code: from typing import Any, Dict, List, Optional, Tuple, Type, Union
  
  import gym
  import numpy as np
  import torch as th
  from torch.nn import functional as F
  
  from stable_baselines3.common.buffers import ReplayBuffer
  from stable_baselines3.common.noise import ActionNoise
  from stable_baselines3.common.off_policy_algorithm import OffPolicyAlgorithm
  from stable_baselines3.common.type_aliases import GymEnv, MaybeCallback, Schedule
  from stable_baselines3.common.utils import polyak_update
  from stable_baselines3.sac.policies import SACPolicy
  
  
  class SAC(OffPolicyAlgorithm):
      """
      Soft Actor-Critic (SAC)
      Off-Policy Maximum Entropy Deep Reinforcement Learning with a Stochastic Actor,
      This implementation borrows code from original implementation (https://github.com/haarnoja/sac)
      from OpenAI Spinning Up (https://github.com/openai/spinningup), from the softlearning repo
      (https://github.com/rail-berkeley/softlearning/)
      and from Stable Baselines (https://github.com/hill-a/stable-baselines)
      Paper: https://arxiv.org/abs/1801.01290
      Introduction to SAC: https://spinningup.openai.com/en/latest/algorithms/sac.html
  
      Note: we use double q target and not value target as discussed
      in https://github.com/hill-a/stable-baselines/issues/270
  
      :param policy: The policy model to use (MlpPolicy, CnnPolicy, ...)
      :param env: The environment to learn from (if registered in Gym, can be str)
      :param learning_rate: learning rate for adam optimizer,
          the same learning rate will be used for all networks (Q-Values, Actor and Value function)
          it can be a function of the current progress remaining (from 1 to 0)
      :param buffer_size: size of the replay buffer
      :param learning_starts: how many steps of the model to collect transitions for before learning starts
      :param batch_size: Minibatch size for each gradient update
      :param tau: the soft update coefficient ("Polyak update", between 0 and 1)
      :param gamma: the discount factor
      :param train_freq: Update the model every ``train_freq`` steps. Alternatively pass a tuple of frequency and unit
          like ``(5, "step")`` or ``(2, "episode")``.
      :param gradient_steps: How many gradient steps to do after each rollout (see ``train_freq``)
          Set to ``-1`` means to do as many gradient steps as steps done in the environment
          during the rollout.
      :param action_noise: the action noise type (None by default), this can help
          for hard exploration problem. Cf common.noise for the different action noise type.
      :param replay_buffer_class: Replay buffer class to use (for instance ``HerReplayBuffer``).
          If ``None``, it will be automatically selected.
      :param replay_buffer_kwargs: Keyword arguments to pass to the replay buffer on creation.
      :param optimize_memory_usage: Enable a memory efficient variant of the replay buffer
          at a cost of more complexity.
          See https://github.com/DLR-RM/stable-baselines3/issues/37#issuecomment-637501195
      :param ent_coef: Entropy regularization coefficient. (Equivalent to
          inverse of reward scale in the original SAC paper.)  Controlling exploration/exploitation trade-off.
          Set it to 'auto' to learn it automatically (and 'auto_0.1' for using 0.1 as initial value)
      :param target_update_interval: update the target network every ``target_network_update_freq``
          gradient steps.
      :param target_entropy: target entropy when learning ``ent_coef`` (``ent_coef = 'auto'``)
      :param use_sde: Whether to use generalized State Dependent Exploration (gSDE)
          instead of action noise exploration (default: False)
      :param sde_sample_freq: Sample a new noise matrix every n steps when using gSDE
          Default: -1 (only sample at the beginning of the rollout)
      :param use_sde_at_warmup: Whether to use gSDE instead of uniform sampling
          during the warm up phase (before learning starts)
      :param create_eval_env: Whether to create a second environment that will be
          used for evaluating the agent periodically. (Only available when passing string for the environment)
      :param policy_kwargs: additional arguments to be passed to the policy on creation
      :param verbose: the verbosity level: 0 no output, 1 info, 2 debug
      :param seed: Seed for the pseudo random generators
      :param device: Device (cpu, cuda, ...) on which the code should be run.
          Setting it to auto, the code will be run on the GPU if possible.
      :param _init_setup_model: Whether or not to build the network at the creation of the instance
      """
  
      def __init__(
          self,
          policy: Union[str, Type[SACPolicy]],
          env: Union[GymEnv, str],
          learning_rate: Union[float, Schedule] = 3e-4,
          buffer_size: int = 1_000_000,  # 1e6
          learning_starts: int = 100,
          batch_size: int = 256,
          tau: float = 0.005,
          gamma: float = 0.99,
          train_freq: Union[int, Tuple[int, str]] = 1,
          gradient_steps: int = 1,
          action_noise: Optional[ActionNoise] = None,
          replay_buffer_class: Optional[ReplayBuffer] = None,
          replay_buffer_kwargs: Optional[Dict[str, Any]] = None,
          optimize_memory_usage: bool = False,
          ent_coef: Union[str, float] = "auto",
          target_update_interval: int = 1,
          target_entropy: Union[str, float] = "auto",
          use_sde: bool = False,
          sde_sample_freq: int = -1,
          use_sde_at_warmup: bool = False,
          tensorboard_log: Optional[str] = None,
          create_eval_env: bool = False,
          policy_kwargs: Optional[Dict[str, Any]] = None,
          verbose: int = 0,
          seed: Optional[int] = None,
          device: Union[th.device, str] = "auto",
          _init_setup_model: bool = True,
      ):
  
          super(SAC, self).__init__(
              policy,
              env,
              SACPolicy,
              learning_rate,
              buffer_size,
              learning_starts,
              batch_size,
              tau,
              gamma,
              train_freq,
              gradient_steps,
              action_noise,
              replay_buffer_class=replay_buffer_class,
              replay_buffer_kwargs=replay_buffer_kwargs,
              policy_kwargs=policy_kwargs,
              tensorboard_log=tensorboard_log,
              verbose=verbose,
              device=device,
              create_eval_env=create_eval_env,
              seed=seed,
              use_sde=use_sde,
              sde_sample_freq=sde_sample_freq,
              use_sde_at_warmup=use_sde_at_warmup,
              optimize_memory_usage=optimize_memory_usage,
              supported_action_spaces=(gym.spaces.Box),
          )
  
          self.target_entropy = target_entropy
          self.log_ent_coef = None  # type: Optional[th.Tensor]
          # Entropy coefficient / Entropy temperature
          # Inverse of the reward scale
          self.ent_coef = ent_coef
          self.target_update_interval = target_update_interval
          self.ent_coef_optimizer = None
  
          if _init_setup_model:
              self._setup_model()
  
      def _setup_model(self) -> None:
          super(SAC, self)._setup_model()
          self._create_aliases()
          # Target entropy is used when learning the entropy coefficient
          if self.target_entropy == "auto":
              # automatically set target entropy if needed
              self.target_entropy = -np.prod(self.env.action_space.shape).astype(np.float32)
          else:
              # Force conversion
              # this will also throw an error for unexpected string
              self.target_entropy = float(self.target_entropy)
  
          # The entropy coefficient or entropy can be learned automatically
          # see Automating Entropy Adjustment for Maximum Entropy RL section
          # of https://arxiv.org/abs/1812.05905
          if isinstance(self.ent_coef, str) and self.ent_coef.startswith("auto"):
              # Default initial value of ent_coef when learned
              init_value = 1.0
              if "_" in self.ent_coef:
                  init_value = float(self.ent_coef.split("_")[1])
                  assert init_value > 0.0, "The initial value of ent_coef must be greater than 0"
  
              # Note: we optimize the log of the entropy coeff which is slightly different from the paper
              # as discussed in https://github.com/rail-berkeley/softlearning/issues/37
              self.log_ent_coef = th.log(th.ones(1, device=self.device) * init_value).requires_grad_(True)
              self.ent_coef_optimizer = th.optim.Adam([self.log_ent_coef], lr=self.lr_schedule(1))
          else:
              # Force conversion to float
              # this will throw an error if a malformed string (different from 'auto')
              # is passed
              self.ent_coef_tensor = th.tensor(float(self.ent_coef)).to(self.device)
  
      def _create_aliases(self) -> None:
          self.actor = self.policy.actor
          self.critic = self.policy.critic
          self.critic_target = self.policy.critic_target
  
      def train(self, gradient_steps: int, batch_size: int = 64) -> None:
          # Switch to train mode (this affects batch norm / dropout)
          self.policy.set_training_mode(True)
          # Update optimizers learning rate
          optimizers = [self.actor.optimizer, self.critic.optimizer]
          if self.ent_coef_optimizer is not None:
              optimizers += [self.ent_coef_optimizer]
  
          # Update learning rate according to lr schedule
          self._update_learning_rate(optimizers)
  
          ent_coef_losses, ent_coefs = [], []
          actor_losses, critic_losses = [], []
  
          for gradient_step in range(gradient_steps):
              # Sample replay buffer
              replay_data = self.replay_buffer.sample(batch_size, env=self._vec_normalize_env)
  
              # We need to sample because `log_std` may have changed between two gradient steps
              if self.use_sde:
                  self.actor.reset_noise()
  
              # Action by the current actor for the sampled state
              actions_pi, log_prob = self.actor.action_log_prob(replay_data.observations)
              log_prob = log_prob.reshape(-1, 1)
  
              ent_coef_loss = None
              if self.ent_coef_optimizer is not None:
                  # Important: detach the variable from the graph
                  # so we don't change it with other losses
                  # see https://github.com/rail-berkeley/softlearning/issues/60
                  ent_coef = th.exp(self.log_ent_coef.detach())
                  ent_coef_loss = -(self.log_ent_coef * (log_prob + self.target_entropy).detach()).mean()
                  ent_coef_losses.append(ent_coef_loss.item())
              else:
                  ent_coef = self.ent_coef_tensor
  
              ent_coefs.append(ent_coef.item())
  
              # Optimize entropy coefficient, also called
              # entropy temperature or alpha in the paper
              if ent_coef_loss is not None:
                  self.ent_coef_optimizer.zero_grad()
                  ent_coef_loss.backward()
                  self.ent_coef_optimizer.step()
  
              with th.no_grad():
                  # Select action according to policy
                  next_actions, next_log_prob = self.actor.action_log_prob(replay_data.next_observations)
                  # Compute the next Q values: min over all critics targets
                  next_q_values = th.cat(self.critic_target(replay_data.next_observations, next_actions), dim=1)
                  next_q_values, _ = th.min(next_q_values, dim=1, keepdim=True)
                  # add entropy term
                  next_q_values = next_q_values - ent_coef * next_log_prob.reshape(-1, 1)
                  # td error + entropy term
                  target_q_values = replay_data.rewards + (1 - replay_data.dones) * self.gamma * next_q_values
  
              # Get current Q-values estimates for each critic network
              # using action from the replay buffer
              current_q_values = self.critic(replay_data.observations, replay_data.actions)
  
              # Compute critic loss
              critic_loss = 0.5 * sum([F.mse_loss(current_q, target_q_values) for current_q in current_q_values])
              critic_losses.append(critic_loss.item())
  
              # Optimize the critic
              self.critic.optimizer.zero_grad()
              critic_loss.backward()
              self.critic.optimizer.step()
  
              # Compute actor loss
              # Alternative: actor_loss = th.mean(log_prob - qf1_pi)
              # Mean over all critic networks
              q_values_pi = th.cat(self.critic.forward(replay_data.observations, actions_pi), dim=1)
              min_qf_pi, _ = th.min(q_values_pi, dim=1, keepdim=True)
              actor_loss = (ent_coef * log_prob - min_qf_pi).mean()
              actor_losses.append(actor_loss.item())
  
              # Optimize the actor
              self.actor.optimizer.zero_grad()
              actor_loss.backward()
              self.actor.optimizer.step()
  
              # Update target networks
              if gradient_step % self.target_update_interval == 0:
                  polyak_update(self.critic.parameters(), self.critic_target.parameters(), self.tau)
  
          self._n_updates += gradient_steps
  
          self.logger.record("train/n_updates", self._n_updates, exclude="tensorboard")
          self.logger.record("train/ent_coef", np.mean(ent_coefs))
          self.logger.record("train/actor_loss", np.mean(actor_losses))
          self.logger.record("train/critic_loss", np.mean(critic_losses))
          if len(ent_coef_losses) > 0:
              self.logger.record("train/ent_coef_loss", np.mean(ent_coef_losses))
  
      def learn(
          self,
          total_timesteps: int,
          callback: MaybeCallback = None,
          log_interval: int = 4,
          eval_env: Optional[GymEnv] = None,
          eval_freq: int = -1,
          n_eval_episodes: int = 5,
          tb_log_name: str = "SAC",
          eval_log_path: Optional[str] = None,
          reset_num_timesteps: bool = True,
          render: Optional[bool] = False,
      ) -> OffPolicyAlgorithm:
  
          return super(SAC, self).learn(
              total_timesteps=total_timesteps,
              callback=callback,
              log_interval=log_interval,
              eval_env=eval_env,
              eval_freq=eval_freq,
              n_eval_episodes=n_eval_episodes,
              tb_log_name=tb_log_name,
              eval_log_path=eval_log_path,
              reset_num_timesteps=reset_num_timesteps,
              render=render,
          )
  
      def _excluded_save_params(self) -> List[str]:
          return super(SAC, self)._excluded_save_params() + ["actor", "critic", "critic_target"]
  
      def _get_torch_save_params(self) -> Tuple[List[str], List[str]]:
          state_dicts = ["policy", "actor.optimizer", "critic.optimizer"]
          if self.ent_coef_optimizer is not None:
              saved_pytorch_variables = ["log_ent_coef"]
              state_dicts.append("ent_coef_optimizer")
          else:
              saved_pytorch_variables = ["ent_coef_tensor"]
          return state_dicts, saved_pytorch_variables
  
apis: torch.nn.functional.mse_loss torch.min torch.ones torch.no_grad numpy.prod numpy.mean