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有没有想过强化学习 (RL) 是如何工作的？

在本文中，我们将从头开始构建最简单的强化学习形式之一 —普通策略梯度（VPG）算法。 然后，我们将训练它完成著名的 CartPole 挑战 — 学习从左向右移动购物车以平衡杆子。 在此过程中，我们还将完成对 OpenAI 的 Spinning Up 学习资源的第一个挑战。

本文的代码可以在 这里 找到。

1、我们的方法

我们将通过创建一个简单的深度学习模型来解决这个问题，该模型接受观察并输出随机策略（即采取每个可能行动的概率）。

然后，我们需要做的就是通过在环境中采取行动并使用此策略来收集经验。

当我们有足够的批量经验（几个episode经验的集合）后，我们需要转向梯度下降来改进模型。 在较高层面上，我们希望增加策略的预期回报，这意味着调整权重和偏差以增加高预期回报行动的概率。 就 VPG 而言，这意味着使用策略梯度定理，该定理给出了该预期回报的梯度方程（如下所示）。

这就是全部内容了—所以让我们开始编码吧！

2、创建模型

我们将首先创建一个带有一个隐藏层的非常简单的模型。 第一个线性层从 CartPole 的观察空间获取输入特征，最后一层返回可能结果的值。

def create_model(number_observation_features: int, number_actions: int) -> nn.Module:
    """Create the MLP model

    Args:
        number_observation_features (int): Number of features in the (flat)
        observation tensor
        number_actions (int): Number of actions

    Returns:
        nn.Module: Simple MLP model
    """
    hidden_layer_features = 32

    return nn.Sequential(
        nn.Linear(in_features=number_observation_features,
                  out_features=hidden_layer_features),
        nn.ReLU(),
        nn.Linear(in_features=hidden_layer_features,
                  out_features=number_actions),
    )

3、获取策略

我们还需要为每个时间步获取一个模型策略（以便我们知道如何采取行动）。 为此，我们将创建一个 get_policy 函数，该函数使用模型输出策略下每个操作的概率。 然后，我们可以返回一个分类（多项式）分布，该分布可用于选择根据这些概率随机分布的特定动作。

def get_policy(model: nn.Module, observation: np.ndarray) -> Categorical:
    """Get the policy from the model, for a specific observation

    Args:
        model (nn.Module): MLP model
        observation (np.ndarray): Environment observation

    Returns:
        Categorical: Multinomial distribution parameterized by model logits
    """
    observation_tensor = torch.as_tensor(observation, dtype=torch.float32)
    logits = model(observation_tensor)

    # Categorical will also normalize the logits for us
    return Categorical(logits=logits)

4、从策略中采样动作

从这个分类分布中，对于每个时间步长，我们可以对其进行采样以返回一个动作。 我们还将获得该动作的对数概率，这在稍后计算梯度时会很有用。

def get_action(policy: Categorical) -> tuple[int, float]:
    """Sample an action from the policy

    Args:
        policy (Categorical): Policy

    Returns:
        tuple[int, float]: Tuple of the action and it's log probability
    """
    action = policy.sample()  # Unit tensor

    # Converts to an int, as this is what Gym environments require
    action_int = action.item()

    # Calculate the log probability of the action, which is required for
    # calculating the loss later
    log_probability_action = policy.log_prob(action)

    return action_int, log_probability_action

5、计算损失

梯度的完整推导如这里所示。 宽松地说，它是每个状态-动作对的对数概率之和乘以该对所属的整个轨迹的回报的梯度。 额外的外层和汇总若干个情节（即一批），因此我们有重要的数据。

要使用 PyTorch 计算此值，我们可以做的是计算下面的伪损失，然后使用 .backward() 获取上面的梯度（注意我们刚刚删除了梯度项）：

这通常被称为损失，但它并不是真正的损失，因为它不依赖于模型的性能。 它只是对于获取策略梯度有用。

def calculate_loss(epoch_log_probability_actions: torch.Tensor, epoch_action_rewards: torch.Tensor) -> float:
    """Calculate the 'loss' required to get the policy gradient

    Formula for gradient at
    https://spinningup.openai.com/en/latest/spinningup/rl_intro3.html#deriving-the-simplest-policy-gradient

    Note that this isn't really loss - it's just the sum of the log probability
    of each action times the episode return. We calculate this so we can
    back-propagate to get the policy gradient.

    Args:
        epoch_log_probability_actions (torch.Tensor): Log probabilities of the
            actions taken
        epoch_action_rewards (torch.Tensor): Rewards for each of these actions

    Returns:
        float: Pseudo-loss
    """
    return -(epoch_log_probability_actions * epoch_action_rewards).mean()

6、单个epoch训练

将以上所有内容放在一起，我们现在准备好训练一个epoch了。 为此，我们只需循环播放情节（episode）即可创建批次。 在每个情节中，创建一系列可用于训练模型的动作和奖励（即经验）。

def train_one_epoch(env: gym.Env, model: nn.Module, optimizer: Optimizer, max_timesteps=5000, episode_timesteps=200) -> float:
    """Train the model for one epoch

    Args:
        env (gym.Env): Gym environment
        model (nn.Module): Model
        optimizer (Optimizer): Optimizer
        max_timesteps (int, optional): Max timesteps per epoch. Note if an
            episode is part-way through, it will still complete before finishing
            the epoch. Defaults to 5000.
        episode_timesteps (int, optional): Timesteps per episode. Defaults to 200.

    Returns:
        float: Average return from the epoch
    """
    epoch_total_timesteps = 0

    # Returns from each episode (to keep track of progress)
    epoch_returns: list[int] = []

    # Action log probabilities and rewards per step (for calculating loss)
    epoch_log_probability_actions = []
    epoch_action_rewards = []

    # Loop through episodes
    while True:

        # Stop if we've done over the total number of timesteps
        if epoch_total_timesteps > max_timesteps:
            break

        # Running total of this episode's rewards
        episode_reward: int = 0

        # Reset the environment and get a fresh observation
        observation = env.reset()

        # Loop through timesteps until the episode is done (or the max is hit)
        for timestep in range(episode_timesteps):
            epoch_total_timesteps += 1

            # Get the policy and act
            policy = get_policy(model, observation)
            action, log_probability_action = get_action(policy)
            observation, reward, done, _ = env.step(action)

            # Increment the episode rewards
            episode_reward += reward

            # Add epoch action log probabilities
            epoch_log_probability_actions.append(log_probability_action)

            # Finish the action loop if this episode is done
            if done == True:
                # Add one reward per timestep
                for _ in range(timestep + 1):
                    epoch_action_rewards.append(episode_reward)

                break

        # Increment the epoch returns
        epoch_returns.append(episode_reward)

    # Calculate the policy gradient, and use it to step the weights & biases
    epoch_loss = calculate_loss(torch.stack(
        epoch_log_probability_actions),
        torch.as_tensor(
        epoch_action_rewards, dtype=torch.float32)
    )

    epoch_loss.backward()
    optimizer.step()
    optimizer.zero_grad()

    return np.mean(epoch_returns)

7、运行算法

现在可以运行算法了。 

def train(epochs=40) -> None:
    """Train a Vanilla Policy Gradient model on CartPole

    Args:
        epochs (int, optional): The number of epochs to run for. Defaults to 50.
    """

    # Create the Gym Environment
    env = gym.make('CartPole-v0')

    # Use random seeds (to make experiments deterministic)
    torch.manual_seed(0)
    env.seed(0)

    # Create the MLP model
    number_observation_features = env.observation_space.shape[0]
    number_actions = env.action_space.n
    model = create_model(number_observation_features, number_actions)

    # Create the optimizer
    optimizer = Adam(model.parameters(), 1e-2)

    # Loop for each epoch
    for epoch in range(epochs):
        average_return = train_one_epoch(env, model, optimizer)
        print('epoch: %3d \t return: %.3f' % (epoch, average_return))


if __name__ == '__main__':
    train()

大约 40 个 epoch 后，可以看到模型已经很好地学习了环境（得分 180+/ 200）：

epoch:  26       return: 118.070
epoch:  27       return: 114.659
epoch:  28       return: 135.405
epoch:  29       return: 144.000
epoch:  30       return: 143.972
epoch:  31       return: 152.091
epoch:  32       return: 166.065
epoch:  33       return: 162.613
epoch:  34       return: 166.806
epoch:  35       return: 172.933
epoch:  36       return: 173.241
epoch:  37       return: 181.071
epoch:  38       return: 186.222
epoch:  39       return: 176.793

原文链接：Vanilla Policy Gradient from Scratch

BimAnt翻译整理，转载请标明出处