[rllib] Fix truncate episodes mode in central critic example (#8073)

rllib/examples/centralized_critic.py

@@ -96,8 +96,6 @@ def centralized_critic_postprocessing(policy,
                                       other_agent_batches=None,
                                       episode=None):
     if policy.loss_initialized():
-        assert sample_batch["dones"][-1], \
-            "Not implemented for train_batch_mode=truncate_episodes"
         assert other_agent_batches is not None
         [(_, opponent_batch)] = list(other_agent_batches.values())
 
@@ -116,11 +114,17 @@ def centralized_critic_postprocessing(policy,
         sample_batch[OPPONENT_ACTION] = np.zeros_like(
             sample_batch[SampleBatch.ACTIONS])
         sample_batch[SampleBatch.VF_PREDS] = np.zeros_like(
-            sample_batch[SampleBatch.ACTIONS], dtype=np.float32)
+            sample_batch[SampleBatch.REWARDS], dtype=np.float32)
+
+    completed = sample_batch["dones"][-1]
+    if completed:
+        last_r = 0.0
+    else:
+        last_r = sample_batch[SampleBatch.VF_PREDS][-1]
 
     train_batch = compute_advantages(
         sample_batch,
-        0.0,
+        last_r,
         policy.config["gamma"],
         policy.config["lambda"],
         use_gae=policy.config["use_gae"])