动态图机制-DyGraph
- 设置和基本用法
- 基于DyGraph构建网络
- 使用DyGraph训练模型
- 模型参数的保存
- 模型评估
- 编写兼容的模型

动态图机制-DyGraph

PaddlePaddle的DyGraph模式是一种动态的图执行机制，可以立即执行结果，无需构建整个图。同时，和以往静态的执行计算图不同，DyGraph模式下您的所有操作可以立即获得执行结果，而不必等待所构建的计算图全部执行完成，这样可以让您更加直观地构建PaddlePaddle下的深度学习任务，以及进行模型的调试，同时还减少了大量用于构建静态计算图的代码，使得您编写、调试网络的过程变得更加便捷。

PaddlePaddle DyGraph是一个更加灵活易用的模式，可提供：

更加灵活便捷的代码组织结构：使用python的执行控制流程和面向对象的模型设计
更加便捷的调试功能：直接使用python的打印方法即时打印所需要的结果，从而检查正在运行的模型结果便于测试更改
和静态执行图通用的模型代码：同样的模型代码可以使用更加便捷的DyGraph调试，执行，同时也支持使用原有的静态图模式执行

设置和基本用法

升级到最新的PaddlePaddle 1.5:

pip install -q --upgrade paddlepaddle==1.5

使用fluid.dygraph.guard(place=None) 上下文：

import paddle.fluid as fluid
with fluid.dygraph.guard():
    # write your executable dygraph code here

现在您就可以在fluid.dygraph.guard()上下文环境中使用DyGraph的模式运行网络了，DyGraph将改变以往PaddlePaddle的执行方式：现在他们将会立即执行，并且将计算结果返回给Python。

Dygraph将非常适合和Numpy一起使用，使用fluid.dygraph.to_variable(x)将会将ndarray转换为fluid.Variable，而使用fluid.Variable.numpy()将可以把任意时刻获取到的计算结果转换为Numpyndarray：

    x = np.ones([2, 2], np.float32)
    with fluid.dygraph.guard():
        inputs = []
        for _ in range(10):
            inputs.append(fluid.dygraph.to_variable(x))
        ret = fluid.layers.sums(inputs)
        print(ret.numpy())
    [[10. 10.]
    [10. 10.]]
    Process finished with exit code 0

这里创建了一系列ndarray的输入，执行了一个sum操作之后，我们可以直接将运行的结果打印出来

然后通过调用reduce_sum后使用Variable.backward()方法执行反向，使用Variable.gradient()方法即可获得反向网络执行完成后的梯度值的ndarray形式：

    loss = fluid.layers.reduce_sum(ret)
    loss.backward()
    print(loss.gradient())

得到输出：

    [1.]
    Process finished with exit code 0

基于DyGraph构建网络

编写一段用于DyGraph执行的Object-Oriented-Designed, PaddlePaddle模型代码主要由以下三个部分组成： 请注意，如果您设计的这一层结构是包含参数的，则必须要使用继承自fluid.dygraph.Layer的Object-Oriented-Designed的类来描述该层的行为。
- 建立一个可以在DyGraph模式中执行的，Object-Oriented的网络，需要继承自fluid.dygraph.Layer，其中需要调用基类的init方法，并且实现带有参数namescope（用来标识本层的名字）的_init构造函数，在构造函数中，我们通常会执行一些例如参数初始化，子网络初始化的操作，执行这些操作时不依赖于输入的动态信息:

class MyLayer(fluid.dygraph.Layer):
    def __init__(self, name_scope):
        super(MyLayer, self).__init__(name_scope)

实现一个forward(self, *inputs)的执行函数，该函数将负责执行实际运行时网络的执行逻辑，该函数将会在每一轮训练/预测中被调用，这里我们将执行一个简单的relu -> elementwise add -> reduce sum：

def forward(self, inputs):
    x = fluid.layers.relu(inputs)
    self._x_for_debug = x
    x = fluid.layers.elementwise_mul(x, x)
    x = fluid.layers.reduce_sum(x)
    return [x]

在fluid.dygraph.guard()中执行：
- 使用Numpy构建输入：

np_inp = np.array([1.0, 2.0, -1.0], dtype=np.float32)

转换输入的ndarray为Variable, 并执行前向网络获取返回值：使用fluid.dygraph.to_variable(np_inp)转换Numpy输入为DyGraph接收的输入，然后使用my_layer(var_inp)[0]调用callable object并且获取了x作为返回值，利用x.numpy()方法直接获取了执行得到的x的ndarray返回值。

with fluid.dygraph.guard():
    var_inp = fluid.dygraph.to_variable(np_inp)
    my_layer = MyLayer("my_layer")
    x = my_layer(var_inp)[0]
    dy_out = x.numpy()

计算梯度：自动微分对于实现机器学习算法（例如用于训练神经网络的反向传播）来说很有用，使用x.backward()方法可以从某个fluid.Varaible开始执行反向网络，同时利用my_layer._x_for_debug.gradient()获取了网络中x梯度的ndarray 返回值：

x.backward()
dy_grad = my_layer._x_for_debug.gradient()

完整代码如下：

    import paddle.fluid as fluid
    import numpy as np
    class MyLayer(fluid.dygraph.Layer):
        def __init__(self, name_scope):
            super(MyLayer, self).__init__(name_scope)
        def forward(self, inputs):
            x = fluid.layers.relu(inputs)
            self._x_for_debug = x
            x = fluid.layers.elementwise_mul(x, x)
            x = fluid.layers.reduce_sum(x)
            return [x]
    if __name__ == '__main__':
        np_inp = np.array([1.0, 2.0, -1.0], dtype=np.float32)
        with fluid.dygraph.guard():
            var_inp = fluid.dygraph.to_variable(np_inp)
            my_layer = MyLayer("my_layer")
            x = my_layer(var_inp)[0]
            dy_out = x.numpy()
            x.backward()
            dy_grad = my_layer._x_for_debug.gradient()
            my_layer.clear_gradients() # 将参数梯度清零以保证下一轮训练的正确性

使用DyGraph训练模型

接下来我们将以“手写数字识别”这个最基础的模型为例，展示如何利用DyGraph模式搭建并训练一个模型：

有关手写数字识别的相关理论知识请参考PaddleBook中的内容，我们在这里默认您已经了解了该模型所需的深度学习理论知识。

准备数据，我们使用paddle.dataset.mnist作为训练所需要的数据集：

train_reader = paddle.batch(
paddle.dataset.mnist.train(), batch_size=BATCH_SIZE, drop_last=True)

构建网络，虽然您可以根据之前的介绍自己定义所有的网络结构，但是您也可以直接使用fluid.dygraph.Layer当中我们为您定制好的一些基础网络结构，这里我们利用fluid.dygraph.Conv2D以及fluid.dygraph.Pool2d构建了基础的SimpleImgConvPool：

class SimpleImgConvPool(fluid.dygraph.Layer):
    def __init__(self,
                 name_scope,
                 num_filters,
                 filter_size,
                 pool_size,
                 pool_stride,
                 pool_padding=0,
                 pool_type='max',
                 global_pooling=False,
                 conv_stride=1,
                 conv_padding=0,
                 conv_dilation=1,
                 conv_groups=1,
                 act=None,
                 use_cudnn=False,
                 param_attr=None,
                 bias_attr=None):
        super(SimpleImgConvPool, self).__init__(name_scope)
        self._conv2d = fluid.dygraph.Conv2D(
            self.full_name(),
            num_filters=num_filters,
            filter_size=filter_size,
            stride=conv_stride,
            padding=conv_padding,
            dilation=conv_dilation,
            groups=conv_groups,
            param_attr=None,
            bias_attr=None,
            act=act,
            use_cudnn=use_cudnn)
        self._pool2d = fluid.dygraph.Pool2D(
            self.full_name(),
            pool_size=pool_size,
            pool_type=pool_type,
            pool_stride=pool_stride,
            pool_padding=pool_padding,
            global_pooling=global_pooling,
            use_cudnn=use_cudnn)
    def forward(self, inputs):
        x = self._conv2d(inputs)
        x = self._pool2d(x)
        return x

注意: 构建网络时子网络的定义和使用请在init中进行，而子网络的执行则在forward函数中进行

利用已经构建好的SimpleImgConvPool组成最终的MNIST网络：

class MNIST(fluid.dygraph.Layer):
    def __init__(self, name_scope):
        super(MNIST, self).__init__(name_scope)
        self._simple_img_conv_pool_1 = SimpleImgConvPool(
            self.full_name(), 20, 5, 2, 2, act="relu")
        self._simple_img_conv_pool_2 = SimpleImgConvPool(
            self.full_name(), 50, 5, 2, 2, act="relu")
        pool_2_shape = 50 * 4 * 4
        SIZE = 10
        scale = (2.0 / (pool_2_shape**2 * SIZE))**0.5
        self._fc = fluid.dygraph.FC(self.full_name(),
                                    10,
                                    param_attr=fluid.param_attr.ParamAttr(
                                        initializer=fluid.initializer.NormalInitializer(
                                            loc=0.0, scale=scale)),
                                    act="softmax")
    def forward(self, inputs, label=None):
        x = self._simple_img_conv_pool_1(inputs)
        x = self._simple_img_conv_pool_2(x)
        x = self._fc(x)
        if label is not None:
            acc = fluid.layers.accuracy(input=x, label=label)
            return x, acc
        else:
            return x

在fluid.dygraph.guard()中定义配置好的MNIST网络结构，此时即使没有训练也可以在fluid.dygraph.guard()中调用模型并且检查输出：

    with fluid.dygraph.guard():
        mnist = MNIST("mnist")
        id, data = list(enumerate(train_reader()))[0]
        dy_x_data = np.array(
            [x[0].reshape(1, 28, 28)
             for x in data]).astype('float32')
        img = fluid.dygraph.to_variable(dy_x_data)
        print("result is: {}".format(mnist(img).numpy()))
        result is: [[0.10135901 0.1051138  0.1027941  ... 0.0972859  0.10221873 0.10165327]
        [0.09735426 0.09970362 0.10198303 ... 0.10134517 0.10179105 0.10025002]
        [0.09539858 0.10213123 0.09543551 ... 0.10613529 0.10535969 0.097991  ]
        ...
        [0.10120598 0.0996111  0.10512722 ... 0.10067689 0.10088114 0.10071224]
        [0.09889644 0.10033772 0.10151272 ... 0.10245881 0.09878646 0.101483  ]
        [0.09097178 0.10078511 0.10198414 ... 0.10317434 0.10087223 0.09816764]]
        Process finished with exit code 0

构建训练循环，在每一轮参数更新完成后我们调用mnist.clear_gradients()来重置梯度：

with fluid.dygraph.guard():
    epoch_num = 5       
    BATCH_SIZE = 64
    train_reader = paddle.batch(
        paddle.dataset.mnist.train(), batch_size=32, drop_last=True)
    mnist = MNIST("mnist")
    id, data = list(enumerate(train_reader()))[0]
    adam = fluid.optimizer.AdamOptimizer(learning_rate=0.001)
    for epoch in range(epoch_num):
        for batch_id, data in enumerate(train_reader()):
            dy_x_data = np.array([x[0].reshape(1, 28, 28)
                                  for x in data]).astype('float32')
            y_data = np.array(
                [x[1] for x in data]).astype('int64').reshape(-1, 1)
            img = fluid.dygraph.to_variable(dy_x_data)
            label = fluid.dygraph.to_variable(y_data)
            cost = mnist(img)
            loss = fluid.layers.cross_entropy(cost, label)
            avg_loss = fluid.layers.mean(loss)
            if batch_id % 100 == 0 and batch_id is not 0:
                print("epoch: {}, batch_id: {}, loss is: {}".format(epoch, batch_id, avg_loss.numpy()))
            avg_loss.backward()
            adam.minimize(avg_loss)
            mnist.clear_gradients()

变量及优化器

模型的参数或者任何您希望检测的值可以作为变量封装在类中，然后通过对象获取并使用numpy()方法获取其ndarray的输出，在训练过程中您可以使用mnist.parameters()来获取到网络中所有的参数，也可以指定某一个Layer的某个参数或者parameters()来获取该层的所有参数，使用numpy()方法随时查看参数的值

反向运行后调用之前定义的Adam优化器对象的minimize方法进行参数更新:

with fluid.dygraph.guard():
    epoch_num = 5
    BATCH_SIZE = 64
    mnist = MNIST("mnist")
    adam = fluid.optimizer.AdamOptimizer(learning_rate=0.001)
    train_reader = paddle.batch(
        paddle.dataset.mnist.train(), batch_size= BATCH_SIZE, drop_last=True)
    np.set_printoptions(precision=3, suppress=True)
    for epoch in range(epoch_num):
        for batch_id, data in enumerate(train_reader()):
            dy_x_data = np.array(
                [x[0].reshape(1, 28, 28)
                 for x in data]).astype('float32')
            y_data = np.array(
                [x[1] for x in data]).astype('int64').reshape(BATCH_SIZE, 1)
            img = fluid.dygraph.to_variable(dy_x_data)
            label = fluid.dygraph.to_variable(y_data)
            label.stop_gradient = True
            cost = mnist(img)
            loss = fluid.layers.cross_entropy(cost, label)
            avg_loss = fluid.layers.mean(loss)
            dy_out = avg_loss.numpy()
            avg_loss.backward()
            adam.minimize(avg_loss)
            mnist.clear_gradients()
            dy_param_value = {}
            for param in mnist.parameters():
                dy_param_value[param.name] = param.numpy()
            if batch_id % 20 == 0:
                print("Loss at step {}: {}".format(batch_id, avg_loss.numpy()))
    print("Final loss: {}".format(avg_loss.numpy()))
    print("_simple_img_conv_pool_1_conv2d W's mean is: {}".format(mnist._simple_img_conv_pool_1._conv2d._filter_param.numpy().mean()))
    print("_simple_img_conv_pool_1_conv2d Bias's mean is: {}".format(mnist._simple_img_conv_pool_1._conv2d._bias_param.numpy().mean()))
    Loss at step 0: [2.302]
    Loss at step 20: [1.616]
    Loss at step 40: [1.244]
    Loss at step 60: [1.142]
    Loss at step 80: [0.911]
    Loss at step 100: [0.824]
    Loss at step 120: [0.774]
    Loss at step 140: [0.626]
    Loss at step 160: [0.609]
    Loss at step 180: [0.627]
    Loss at step 200: [0.466]
    Loss at step 220: [0.499]
    Loss at step 240: [0.614]
    Loss at step 260: [0.585]
    Loss at step 280: [0.503]
    Loss at step 300: [0.423]
    Loss at step 320: [0.509]
    Loss at step 340: [0.348]
    Loss at step 360: [0.452]
    Loss at step 380: [0.397]
    Loss at step 400: [0.54]
    Loss at step 420: [0.341]
    Loss at step 440: [0.337]
    Loss at step 460: [0.155]
    Final loss: [0.164]
    _simple_img_conv_pool_1_conv2d W's mean is: 0.00606656912714
    _simple_img_conv_pool_1_conv2d Bias's mean is: -3.4576318285e-05

性能

在使用fluid.dygraph.guard()时可以通过传入fluid.CUDAPlace(0)或者fluid.CPUPlace()来选择执行DyGraph的设备，通常如果不做任何处理将会自动适配您的设备。

模型参数的保存

在模型训练中可以使用fluid.dygraph.save_persistables(your_model_object.state_dict(), "save_dir", optimizers=None)来保存your_model_object中所有的模型参数, 以及使用learning rate decay的优化器。也可以自定义需要保存的“参数名” - “参数对象”的Python Dictionary传入。

同样可以使用models，optimizers = fluid.dygraph.load_persistables("save_dir")获取保存的模型参数和优化器。

再使用your_modle_object.load_dict(models)接口来恢复保存的模型参数从而达到继续训练的目的。

以及使用your_optimizer_object.load(optimizers)接口来恢复保存的优化器中的learning rate decay值

下面的代码展示了如何在“手写数字识别”任务中保存参数并且读取已经保存的参数来继续训练。

with fluid.dygraph.guard():
    epoch_num = 5
    BATCH_SIZE = 64
    mnist = MNIST("mnist")
    adam = fluid.optimizer.Adam(learning_rate=0.001)
    train_reader = paddle.batch(
        paddle.dataset.mnist.train(), batch_size= BATCH_SIZE, drop_last=True)
    np.set_printoptions(precision=3, suppress=True)
    dy_param_init_value={}
    for epoch in range(epoch_num):
        for batch_id, data in enumerate(train_reader()):
            dy_x_data = np.array(
                [x[0].reshape(1, 28, 28)
                 for x in data]).astype('float32')
            y_data = np.array(
                [x[1] for x in data]).astype('int64').reshape(BATCH_SIZE, 1)
            img = fluid.dygraph.to_variable(dy_x_data)
            label = fluid.dygraph.to_variable(y_data)
            label.stop_gradient = True
            cost = mnist(img)
            loss = fluid.layers.cross_entropy(cost, label)
            avg_loss = fluid.layers.mean(loss)
            dy_out = avg_loss.numpy()
            avg_loss.backward()
            adam.minimize(avg_loss)
            if batch_id == 20:
                fluid.dygraph.save_persistables(mnist.state_dict(), "save_dir", adam)
            mnist.clear_gradients()
            if batch_id == 20:
                for param in mnist.parameters():
                    dy_param_init_value[param.name] = param.numpy()
                model, _ = fluid.dygraph.load_persistables("save_dir")
                mnist.load_dict(model)
                break
        if epoch == 0:
            break
    restore = mnist.parameters()
    # check save and load
    success = True
    for value in restore:
        if (not np.array_equal(value.numpy(), dy_param_init_value[value.name])) or (not np.isfinite(value.numpy().all())) or (np.isnan(value.numpy().any())):
            success = False
    print("model save and load success? {}".format(success))

模型评估

当我们需要在DyGraph模式下利用搭建的模型进行预测任务，可以使用YourModel.eval()接口，在之前的手写数字识别模型中我们使用mnist.eval()来启动预测模式（我们默认在fluid.dygraph.guard()上下文中是训练模式），在预测的模式下，DyGraph将只会执行前向的预测网络，而不会进行自动求导并执行反向网络：

下面的代码展示了如何使用DyGraph模式训练一个用于执行“手写数字识别”任务的模型并保存，并且利用已经保存好的模型进行预测。

我们在fluid.dygraph.guard()上下文中进行了模型的保存和训练，值得注意的是，当我们需要在训练的过程中进行预测时需要使用YourModel.eval()切换到预测模式，并且在预测完成后使用YourModel.train()切换回训练模式继续训练。

我们在inference_mnist中启用另一个fluid.dygraph.guard()，并在其上下文中load之前保存的checkpoint进行预测，同样的在执行预测前需要使用YourModel.eval()来切换到预测模式。

def test_mnist(reader, model, batch_size):
    acc_set = []
    avg_loss_set = []
    for batch_id, data in enumerate(reader()):
        dy_x_data = np.array([x[0].reshape(1, 28, 28)
                              for x in data]).astype('float32')
        y_data = np.array(
            [x[1] for x in data]).astype('int64').reshape(batch_size, 1)
        img = fluid.dygraph.to_variable(dy_x_data)
        label = fluid.dygraph.to_variable(y_data)
        label.stop_gradient = True
        prediction, acc = model(img, label)
        loss = fluid.layers.cross_entropy(input=prediction, label=label)
        avg_loss = fluid.layers.mean(loss)
        acc_set.append(float(acc.numpy()))
        avg_loss_set.append(float(avg_loss.numpy()))
        # get test acc and loss
    acc_val_mean = np.array(acc_set).mean()
    avg_loss_val_mean = np.array(avg_loss_set).mean()
    return avg_loss_val_mean, acc_val_mean
def inference_mnist():
    with fluid.dygraph.guard():
        mnist_infer = MNIST("mnist")
        # load checkpoint
        model_dict, _ = fluid.dygraph.load_persistables("save_dir")
        mnist_infer.load_dict(model_dict)
        print("checkpoint loaded")
        # start evaluate mode
        mnist_infer.eval()
        def load_image(file):
            im = Image.open(file).convert('L')
            im = im.resize((28, 28), Image.ANTIALIAS)
            im = np.array(im).reshape(1, 1, 28, 28).astype(np.float32)
            im = im / 255.0 * 2.0 - 1.0
            return im
        cur_dir = os.path.dirname(os.path.realpath(__file__))
        tensor_img = load_image(cur_dir + '/image/2.png')
        results = mnist_infer(fluid.dygraph.to_variable(tensor_img))
        lab = np.argsort(results.numpy())
        print("Inference result of image/infer_3.png is: %d" % lab[0][-1])
with fluid.dygraph.guard():
    epoch_num = 1
    BATCH_SIZE = 64
    mnist = MNIST("mnist")
    adam = fluid.optimizer.AdamOptimizer(learning_rate=0.001)
    test_reader = paddle.batch(
        paddle.dataset.mnist.test(), batch_size=BATCH_SIZE, drop_last=True)
    train_reader = paddle.batch(
        paddle.dataset.mnist.train(),
        batch_size=BATCH_SIZE,
        drop_last=True)
    for epoch in range(epoch_num):
        for batch_id, data in enumerate(train_reader()):
            dy_x_data = np.array([x[0].reshape(1, 28, 28)
                                  for x in data]).astype('float32')
            y_data = np.array(
                [x[1] for x in data]).astype('int64').reshape(-1, 1)
            img = fluid.dygraph.to_variable(dy_x_data)
            label = fluid.dygraph.to_variable(y_data)
            label.stop_gradient = True
            cost, acc = mnist(img, label)
            loss = fluid.layers.cross_entropy(cost, label)
            avg_loss = fluid.layers.mean(loss)
            avg_loss.backward()
            adam.minimize(avg_loss)
            # save checkpoint
            mnist.clear_gradients()
            if batch_id % 100 == 0:
                print("Loss at epoch {} step {}: {:}".format(
                    epoch, batch_id, avg_loss.numpy()))
        mnist.eval()
        test_cost, test_acc = test_mnist(test_reader, mnist, BATCH_SIZE)
        mnist.train()
        print("Loss at epoch {} , Test avg_loss is: {}, acc is: {}".format(
            epoch, test_cost, test_acc))
    fluid.dygraph.save_persistables(mnist.state_dict(), "save_dir")
    print("checkpoint saved")
    inference_mnist()
Loss at epoch 0 step 0: [2.2991252]
Loss at epoch 0 step 100: [0.15491392]
Loss at epoch 0 step 200: [0.13315125]
Loss at epoch 0 step 300: [0.10253005]
Loss at epoch 0 step 400: [0.04266362]
Loss at epoch 0 step 500: [0.08894891]
Loss at epoch 0 step 600: [0.08999012]
Loss at epoch 0 step 700: [0.12975612]
Loss at epoch 0 step 800: [0.15257305]
Loss at epoch 0 step 900: [0.07429226]
Loss at epoch 0 , Test avg_loss is: 0.05995981965082674, acc is: 0.9794671474358975
checkpoint saved
No optimizer loaded. If you didn't save optimizer, please ignore this. The program can still work with new optimizer. 
checkpoint loaded
Inference result of image/infer_3.png is: 3

编写兼容的模型

以上一步中手写数字识别的例子为例，动态图的模型代码可以直接用于静态图中作为模型代码，执行时，直接使用PaddlePaddle静态图执行方式即可，这里以静态图中的executor为例, 模型代码可以直接使用之前的模型代码，执行时使用Executor执行即可

epoch_num = 1
BATCH_SIZE = 64
exe = fluid.Executor(fluid.CPUPlace())
mnist = MNIST("mnist")
sgd = fluid.optimizer.SGDOptimizer(learning_rate=1e-3)
train_reader = paddle.batch(
    paddle.dataset.mnist.train(), batch_size=BATCH_SIZE, drop_last=True)
img = fluid.layers.data(
    name='pixel', shape=[1, 28, 28], dtype='float32')
label = fluid.layers.data(name='label', shape=[1], dtype='int64')
cost = mnist(img)
loss = fluid.layers.cross_entropy(cost, label)
avg_loss = fluid.layers.mean(loss)
sgd.minimize(avg_loss)
out = exe.run(fluid.default_startup_program())
for epoch in range(epoch_num):
    for batch_id, data in enumerate(train_reader()):
        static_x_data = np.array(
            [x[0].reshape(1, 28, 28)
             for x in data]).astype('float32')
        y_data = np.array(
            [x[1] for x in data]).astype('int64').reshape([BATCH_SIZE, 1])
        fetch_list = [avg_loss.name]
        out = exe.run(
            fluid.default_main_program(),
            feed={"pixel": static_x_data,
                  "label": y_data},
            fetch_list=fetch_list)
        static_out = out[0]
        if batch_id % 100 == 0 and batch_id is not 0:
            print("epoch: {}, batch_id: {}, loss: {}".format(epoch, batch_id, static_out))