added docstrings to action flattener

ml-agents/mlagents/trainers/policy/tf_policy.py

             reparameterize,
             condition_sigma_on_obs,
         )
-        if self.behavior_spec.action_spec.continuous_size > 0 and self.behavior_spec.action_spec.discrete_size > 0:
+        if (
+            self.behavior_spec.action_spec.continuous_size > 0
+            and self.behavior_spec.action_spec.discrete_size > 0
+        ):
             raise UnityPolicyException(
                 "TensorFlow does not support mixed action spaces. Please run with the Torch framework."
             )

ml-agents/mlagents/trainers/torch/action_flattener.py

 
 class ActionFlattener:
     def __init__(self, action_spec: ActionSpec):
+        """
+        A torch module that creates the flattened form of an AgentAction object.
+        The flattened form is the continuous action concatenated with the
+        concatenated one hot encodings of the discrete actions.
+        :param action_spec: An ActionSpec that describes the action space dimensions
+        """
+        """
+        The flattened size is the continuous size plus the sum of the branch sizes
+        since discrete actions are encoded as one hots.
+        """
+        """
+        Returns a tensor corresponding the flattened action
+        :param action: An AgentAction object
+        """
         action_list: List[torch.Tensor] = []
         if self._specs.continuous_size > 0:
             action_list.append(action.continuous_tensor)