[RLlib] IMPALA PyTorch (#8287)

This PR adds an IMPALA PyTorch implementation.

- adds compilation tests for LSTM and w/o LSTM.
- adds learning test for CartPole.

rllib/utils/exploration/tests/test_explorations.py

@@ -30,10 +30,7 @@ def do_test_explorations(run,
 
     # Test all frameworks.
     for fw in framework_iterator(core_config):
-        if fw == "torch" and \
-                run in [impala.ImpalaTrainer, sac.SACTrainer]:
-            continue
-        elif fw == "eager" and run in [
+        if fw == "eager" and run in [
                 ddpg.DDPGTrainer, sac.SACTrainer, td3.TD3Trainer
         ]:
             continue

rllib/utils/test_utils.py

@@ -47,7 +47,7 @@ def framework_iterator(config=None,
             logger.warning(
                 "framework_iterator skipping torch (not installed)!")
             continue
-        elif not tf:
+        if fw != "torch" and not tf:
             logger.warning("framework_iterator skipping {} (tf not "
                            "installed)!".format(fw))
             continue