PyTorch教程-9.6. 递归神经网络的简洁实现

与我们大多数从头开始的实施一样， 第 9.5 节旨在深入了解每个组件的工作原理。但是，当您每天使用 RNN 或编写生产代码时，您会希望更多地依赖于减少实现时间（通过为通用模型和函数提供库代码）和计算时间（通过优化这些库实现）。本节将向您展示如何使用深度学习框架提供的高级 API 更有效地实现相同的语言模型。和以前一样，我们首先加载时间机器数据集。

import torch
from torch import nn
from torch.nn import functional as F
from d2l import torch as d2l

from mxnet import np, npx
from mxnet.gluon import nn, rnn
from d2l import mxnet as d2l

npx.set_np()

from flax import linen as nn
from jax import numpy as jnp
from d2l import jax as d2l

No GPU/TPU found, falling back to CPU. (Set TF_CPP_MIN_LOG_LEVEL=0 and rerun for more info.)

import tensorflow as tf
from d2l import tensorflow as d2l

9.6.1. 定义模型

我们使用由高级 API 实现的 RNN 定义以下类。

class RNN(d2l.Module): #@save
  """The RNN model implemented with high-level APIs."""
  def __init__(self, num_inputs, num_hiddens):
    super().__init__()
    self.save_hyperparameters()
    self.rnn = nn.RNN(num_inputs, num_hiddens)

  def forward(self, inputs, H=None):
    return self.rnn(inputs, H)

Specifically, to initialize the hidden state, we invoke the member method begin_state. This returns a list that contains an initial hidden state for each example in the minibatch, whose shape is (number of hidden layers, batch size, number of hidden units). For some models to be introduced later (e.g., long short-term memory), this list will also contain other information.

class RNN(d2l.Module): #@save
  """The RNN model implemented with high-level APIs."""
  def __init__(self, num_hiddens):
    super().__init__()
    self.save_hyperparameters()
    self.rnn = rnn.RNN(num_hiddens)

  def forward(self, inputs, H=None):
    if H is None:
      H, = self.rnn.begin_state(inputs.shape[1], ctx=inputs.ctx)
    outputs, (H, ) = self.rnn(inputs, (H, ))
    return outputs, H

Flax does not provide an RNNCell for concise implementation of Vanilla RNNs as of today. There are more advanced variants of RNNs like LSTMs and GRUs which are available in the Flax linen API.

class RNN(nn.Module): #@save
  """The RNN model implemented with high-level APIs."""
  num_hiddens: int

  @nn.compact
  def __call__(self, inputs, H=None):
    raise NotImplementedError

class RNN(d2l.Module): #@save
  """The RNN model implemented with high-level APIs."""
  def __init__(self, num_hiddens):
    super().__init__()
    self.save_hyperparameters()
    self.rnn = tf.keras.layers.SimpleRNN(
      num_hiddens, return_sequences=True, return_state=True,
      time_major=True)

  def forward(self, inputs, H=None):
    outputs, H = self.rnn(inputs, H)
    return outputs, H

继承自9.5 节RNNLMScratch中的类 ，下面的类定义了一个完整的基于 RNN 的语言模型。请注意，我们需要创建一个单独的全连接输出层。RNNLM

class RNNLM(d2l.RNNLMScratch): #@save
  """The RNN-based language model implemented with high-level APIs."""
  def init_params(self):
    self.linear = nn.LazyLinear(self.vocab_size)

  def output_layer(self, hiddens):
    return self.linear(hiddens).swapaxes(0, 1)

class RNNLM(d2l.RNNLMScratch): #@save
  """The RNN-based language model implemented with high-level APIs."""
  def init_params(self):
    self.linear = nn.Dense(self.vocab_size, flatten=False)
    self.initialize()
  def output_layer(self, hiddens):
    return self.linear(hiddens).swapaxes(0, 1)

class RNNLM(d2l.RNNLMScratch): #@save
  """The RNN-based language model implemented with high-level APIs."""
  training: bool = True

  def setup(self):
    self.linear = nn.Dense(self.vocab_size)

  def output_layer(self, hiddens):
    return self.linear(hiddens).swapaxes(0, 1)

  def forward(self, X, state=None):
    embs = self.one_hot(X)
    rnn_outputs, _ = self.rnn(embs, state, self.training)
    return self.output_layer(rnn_outputs)

class RNNLM(d2l.RNNLMScratch): #@save
  """The RNN-based language model implemented with high-level APIs."""
  def init_params(self):
    self.linear = tf.keras.layers.Dense(self.vocab_size)

  def output_layer(self, hiddens):
    return tf.transpose(self.linear(hiddens), (1, 0, 2))

9.6.2. 训练和预测

在训练模型之前，让我们使用随机权重初始化的模型进行预测。鉴于我们还没有训练网络，它会产生无意义的预测。

data = d2l.TimeMachine(batch_size=1024, num_steps=32)
rnn = RNN(num_inputs=len(data.vocab), num_hiddens=32)
model = RNNLM(rnn, vocab_size=len(data.vocab), lr=1)
model.predict('it has', 20, data.vocab)

'it hasgggggggggggggggggggg'

data = d2l.TimeMachine(batch_size=1024, num_steps=32)
rnn = RNN(num_hiddens=32)
model = RNNLM(rnn, vocab_size=len(data.vocab), lr=1)
model.predict('it has', 20, data.vocab)

'it hasxlxlxlxlxlxlxlxlxlxl'

data = d2l.TimeMachine(batch_size=1024, num_steps=32)
rnn = RNN(num_hiddens=32)
model = RNNLM(rnn, vocab_size=len(data.vocab), lr=1)
model.predict('it has', 20, data.vocab)

'it hasnvjdtagwbcsxvcjwuyby'

接下来，我们利用高级 API 训练我们的模型。

trainer = d2l.Trainer(max_epochs=100, gradient_clip_val=1, num_gpus=1)
trainer.fit(model, data)

trainer = d2l.Trainer(max_epochs=100, gradient_clip_val=1, num_gpus=1)
trainer.fit(model, data)

with d2l.try_gpu():
  trainer = d2l.Trainer(max_epochs=100, gradient_clip_val=1)
trainer.fit(model, data)

与第 9.5 节相比，该模型实现了相当的困惑度，但由于实现优化，运行速度更快。和以前一样，我们可以在指定的前缀字符串之后生成预测标记。

model.predict('it has', 20, data.vocab, d2l.try_gpu())

'it has and the time trave '

model.predict('it has', 20, data.vocab, d2l.try_gpu())

'it has and the thi baid th'

model.predict('it has', 20, data.vocab)

'it has our in the time tim'

9.6.3. 概括

深度学习框架中的高级 API 提供标准 RNN 的实现。这些库可帮助您避免浪费时间重新实现标准模型。此外，框架实施通常经过高度优化，与从头开始实施相比，可显着提高（计算）性能。

9.6.4. 练习

您能否使用高级 API 使 RNN 模型过拟合？

使用 RNN实现第 9.1 节的自回归模型。