class_docstr: PyTorch policy class used with Impala.
code: def loss(
      self,
      model: ModelV2,
      dist_class: Type[ActionDistribution],
      train_batch: SampleBatch,
  ) -> Union[TensorType, List[TensorType]]:
      model_out, _ = model(train_batch)
      action_dist = dist_class(model_out, model)
  
      if isinstance(self.action_space, gym.spaces.Discrete):
          is_multidiscrete = False
          output_hidden_shape = [self.action_space.n]
      elif isinstance(self.action_space, gym.spaces.MultiDiscrete):
          is_multidiscrete = True
          output_hidden_shape = self.action_space.nvec.astype(np.int32)
      else:
          is_multidiscrete = False
          output_hidden_shape = 1
  
      def _make_time_major(*args, **kw):
          return make_time_major(
              self, train_batch.get(SampleBatch.SEQ_LENS), *args, **kw
          )
  
      actions = train_batch[SampleBatch.ACTIONS]
      dones = train_batch[SampleBatch.TERMINATEDS]
      rewards = train_batch[SampleBatch.REWARDS]
      behaviour_action_logp = train_batch[SampleBatch.ACTION_LOGP]
      behaviour_logits = train_batch[SampleBatch.ACTION_DIST_INPUTS]
      if isinstance(output_hidden_shape, (list, tuple, np.ndarray)):
          unpacked_behaviour_logits = torch.split(
              behaviour_logits, list(output_hidden_shape), dim=1
          )
          unpacked_outputs = torch.split(
              model_out, list(output_hidden_shape), dim=1
          )
      else:
          unpacked_behaviour_logits = torch.chunk(
              behaviour_logits, output_hidden_shape, dim=1
          )
          unpacked_outputs = torch.chunk(
              model_out, output_hidden_shape, dim=1
          )
      values = model.value_function()
      values_time_major = _make_time_major(values)
      bootstrap_values_time_major = _make_time_major(
          train_batch[SampleBatch.VALUES_BOOTSTRAPPED]
      )
      bootstrap_value = bootstrap_values_time_major[-1]
  
      if self.is_recurrent():
          max_seq_len = torch.max(train_batch[SampleBatch.SEQ_LENS])
          mask_orig = sequence_mask(
              train_batch[SampleBatch.SEQ_LENS], max_seq_len
          )
          mask = torch.reshape(mask_orig, [-1])
      else:
          mask = torch.ones_like(rewards)
  
      # Prepare actions for loss.
      loss_actions = (
          actions
          if is_multidiscrete
          else torch.unsqueeze(actions, dim=1)
      )
  
      # Inputs are reshaped from [B * T] => [(T|T-1), B] for V-trace calc.
      loss = VTraceLoss(
          actions=_make_time_major(loss_actions),
          actions_logp=_make_time_major(action_dist.logp(actions)),
          actions_entropy=_make_time_major(action_dist.entropy()),
          dones=_make_time_major(dones),
          behaviour_action_logp=_make_time_major(behaviour_action_logp),
          behaviour_logits=_make_time_major(unpacked_behaviour_logits),
          target_logits=_make_time_major(unpacked_outputs),
          discount=self.config["gamma"],
          rewards=_make_time_major(rewards),
          values=values_time_major,
          bootstrap_value=bootstrap_value,
          dist_class=TorchCategorical
          if is_multidiscrete
          else dist_class,
          model=model,
          valid_mask=_make_time_major(mask),
          config=self.config,
          vf_loss_coeff=self.config["vf_loss_coeff"],
          entropy_coeff=self.entropy_coeff,
          clip_rho_threshold=self.config["vtrace_clip_rho_threshold"],
          clip_pg_rho_threshold=self.config[
              "vtrace_clip_pg_rho_threshold"
          ],
      )
  
      # Store values for stats function in model (tower), such that for
      # multi-GPU, we do not override them during the parallel loss phase.
      model.tower_stats["pi_loss"] = loss.pi_loss
      model.tower_stats["vf_loss"] = loss.vf_loss
      model.tower_stats["entropy"] = loss.entropy
      model.tower_stats["mean_entropy"] = loss.mean_entropy
      model.tower_stats["total_loss"] = loss.total_loss
  
      values_batched = make_time_major(
          self,
          train_batch.get(SampleBatch.SEQ_LENS),
          values,
      )
      model.tower_stats["vf_explained_var"] = explained_variance(
          torch.reshape(loss.value_targets, [-1]),
          torch.reshape(values_batched, [-1]),
      )
  
      if self.config.get("_separate_vf_optimizer"):
          return loss.loss_wo_vf, loss.vf_loss
      else:
          return loss.total_loss
  
imports: import gymnasium
  import logging
  import numpy
  from typing import Dict, List, Optional, Type, Union
  import ray
  from ray.rllib.evaluation.episode import Episode
  from ray.rllib.evaluation.postprocessing import compute_bootstrap_value
  from ray.rllib.models.modelv2 import ModelV2
  from ray.rllib.models.action_dist import ActionDistribution
  from ray.rllib.models.torch.torch_action_dist import TorchCategorical
  from ray.rllib.policy.sample_batch import SampleBatch
  from ray.rllib.policy.torch_mixins import EntropyCoeffSchedule, LearningRateSchedule, ValueNetworkMixin
  from ray.rllib.policy.torch_policy_v2 import TorchPolicyV2
  from ray.rllib.utils.annotations import override
  from ray.rllib.utils.framework import try_import_torch
  from ray.rllib.utils.numpy import convert_to_numpy
  from ray.rllib.utils.torch_utils import apply_grad_clipping, explained_variance, global_norm, sequence_mask
  from ray.rllib.utils.typing import TensorType
label_desc: Use image node_id 2 for calling the ImpalaTorchPolicy obj's underlying member method code with example usage: obj.loss(model, dist_class, train_batch) and returns: make_time_major, loss,  loss, loss