Fix memory logic in SeparateActorCritic

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

         memories: Optional[torch.Tensor] = None,
         sequence_length: int = 1,
     ) -> Tuple[Dict[str, torch.Tensor], torch.Tensor]:
+        actor_mem, critic_mem = None, None
-        else:
-            critic_mem = None
-        # Make memories with the actor mem unchanged
-        memories_out = torch.cat([actor_mem, critic_mem_out], dim=-1)
+        if actor_mem is not None:
+            # Make memories with the actor mem unchanged
+            memories_out = torch.cat([actor_mem, critic_mem_out], dim=-1)
+        else:
+            memories_out = None
         return value_outputs, memories_out
 
     def get_dist_and_value(