Clear agent processor properly on episode reset (#3437)

ml-agents/mlagents/trainers/agent_processor.py

         Ends the episode, terminating the current trajectory and stopping stats collection for that
         episode. Used for forceful reset (e.g. in curriculum or generalization training.)
         """
-        self.experience_buffers.clear()
-        self.episode_rewards.clear()
-        self.episode_steps.clear()
+        all_gids = list(self.experience_buffers.keys())  # Need to make copy
+        for _gid in all_gids:
+            self._clean_agent_data(_gid)
 
 
 class AgentManagerQueue(Generic[T]):

ml-agents/mlagents/trainers/tests/test_agent_processor.py

     assert len(processor.episode_rewards.keys()) == 0
 
 
+def test_end_episode():
+    policy = create_mock_policy()
+    tqueue = mock.Mock()
+    name_behavior_id = "test_brain_name"
+    processor = AgentProcessor(
+        policy,
+        name_behavior_id,
+        max_trajectory_length=5,
+        stats_reporter=StatsReporter("testcat"),
+    )
+
+    fake_action_outputs = {
+        "action": [0.1],
+        "entropy": np.array([1.0], dtype=np.float32),
+        "learning_rate": 1.0,
+        "pre_action": [0.1],
+        "log_probs": [0.1],
+    }
+    mock_step = mb.create_mock_batchedstep(
+        num_agents=1,
+        num_vector_observations=8,
+        action_shape=[2],
+        num_vis_observations=0,
+    )
+    fake_action_info = ActionInfo(
+        action=[0.1],
+        value=[0.1],
+        outputs=fake_action_outputs,
+        agent_ids=mock_step.agent_id,
+    )
+
+    processor.publish_trajectory_queue(tqueue)
+    # This is like the initial state after the env reset
+    processor.add_experiences(mock_step, 0, ActionInfo.empty())
+    # Run 3 trajectories, with different workers (to simulate different agents)
+    remove_calls = []
+    for _ep in range(3):
+        remove_calls.append(mock.call([get_global_agent_id(_ep, 0)]))
+        for _ in range(5):
+            processor.add_experiences(mock_step, _ep, fake_action_info)
+            # Make sure we don't add experiences from the prior agents after the done
+
+    # Call end episode
+    processor.end_episode()
+    # Check that we removed every agent
+    policy.remove_previous_action.assert_has_calls(remove_calls)
+    # Check that there are no experiences left
+    assert len(processor.experience_buffers.keys()) == 0
+    assert len(processor.last_take_action_outputs.keys()) == 0
+    assert len(processor.episode_steps.keys()) == 0
+    assert len(processor.episode_rewards.keys()) == 0
+
+
 def test_agent_manager():
     policy = create_mock_policy()
     name_behavior_id = "test_brain_name"