[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/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