[rllib] Add helper to iterate over envs in a vectorized environment (#4001)

* add foreach env func

* fix

* add test

python/ray/rllib/evaluation/policy_evaluator.py

@@ -494,6 +494,16 @@ class PolicyEvaluator(EvaluatorInterface):
                 self.policy_map[DEFAULT_POLICY_ID].compute_apply(samples))
             return grad_fetch
 
+    @DeveloperAPI
+    def foreach_env(self, func):
+        """Apply the given function to each underlying env instance."""
+
+        envs = self.async_env.get_unwrapped()
+        if not envs:
+            return [func(self.async_env)]
+        else:
+            return [func(e) for e in envs]
+
     @DeveloperAPI
     def get_policy(self, policy_id=DEFAULT_POLICY_ID):
         """Return policy graph for the specified id, or None.

python/ray/rllib/test/test_policy_evaluator.py

@@ -209,16 +209,21 @@ class TestPolicyEvaluator(unittest.TestCase):
     def testQueryEvaluators(self):
         register_env("test", lambda _: gym.make("CartPole-v0"))
         pg = PGAgent(
-            env="test", config={
+            env="test",
+            config={
                 "num_workers": 2,
-                "sample_batch_size": 5
+                "sample_batch_size": 5,
+                "num_envs_per_worker": 2,
             })
         results = pg.optimizer.foreach_evaluator(
             lambda ev: ev.sample_batch_size)
         results2 = pg.optimizer.foreach_evaluator_with_index(
             lambda ev, i: (i, ev.sample_batch_size))
-        self.assertEqual(results, [5, 5, 5])
-        self.assertEqual(results2, [(0, 5), (1, 5), (2, 5)])
+        results3 = pg.optimizer.foreach_evaluator(
+            lambda ev: ev.foreach_env(lambda env: 1))
+        self.assertEqual(results, [10, 10, 10])
+        self.assertEqual(results2, [(0, 10), (1, 10), (2, 10)])
+        self.assertEqual(results3, [[1, 1], [1, 1], [1, 1]])
 
     def testRewardClipping(self):
         # clipping on