PG unify/cleanup tf vs torch and PG functionality test cases (tf + torch). (#6650)

* Unifying the code for PGTrainer/Policy wrt tf vs torch.
Adding loss function test cases for the PGAgent (confirm equivalence of tf and torch).

* Fix LINT line-len errors.

* Fix LINT errors.

* Fix `tf_pg_policy` imports (formerly: `pg_policy`).

* Rename tf_pg_... into pg_tf_... following <alg>_<framework>_... convention, where ...=policy/loss/agent/trainer.
Retire `PGAgent` class (use PGTrainer instead).

* - Move PG test into agents/pg/tests directory.
- All test cases will be located near the classes that are tested and
  then built into the Bazel/Travis test suite.

* Moved post_process_advantages into pg.py (from pg_tf_policy.py), b/c
the function is not a tf-specific one.

* Fix remaining import errors for agents/pg/...

* Fix circular dependency in pg imports.

* Add pg tests to Jenkins test suite.

python/ray/tune/trainable.py

@@ -124,8 +124,13 @@ class Trainable(object):
 
     @classmethod
     def resource_help(cls, config):
-        """Returns a help string for configuring this trainable's resources."""
+        """
+        Args:
+            config (dict): The Trainer's config dict.
 
+        Returns:
+             str: A help string for configuring this trainable's resources.
+        """
         return ""
 
     def current_ip(self):