fix mock_brain (#2377)

fix mock_brain

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

         buffer[0]["prev_action"].append(current_brain_info.previous_vector_actions[0])
         buffer[0]["masks"].append(1.0)
         buffer[0]["advantages"].append(1.0)
-        buffer[0]["action_probs"].append(np.ones(buffer[0]["actions"][0].shape))
+        if brain_params.vector_action_space_type == "discrete":
+            buffer[0]["action_probs"].append(
+                np.ones(sum(brain_params.vector_action_space_size))
+            )
+        else:
+            buffer[0]["action_probs"].append(np.ones(buffer[0]["actions"][0].shape))
         buffer[0]["actions_pre"].append(np.ones(buffer[0]["actions"][0].shape))
         buffer[0]["random_normal_epsilon"].append(
             np.ones(buffer[0]["actions"][0].shape)