[Tune] Add export_formats option to export policy graphs (#3868)

In earlier PRs, PR#3585 and PR#3637, export_policy_model and export_policy_checkpoint were introduced for users to export TensorFlow model and checkpoint.

For Ray Tune users, these APIs are not accessible through YAML configurations.

In this pull request, export_formats option is provided to enable users to choose the desired export format.

python/ray/tune/experiment.py

@@ -72,6 +72,8 @@ class Experiment(object):
             checkpoints. A value of 0 (default) disables checkpointing.
         checkpoint_at_end (bool): Whether to checkpoint at the end of the
             experiment regardless of the checkpoint_freq. Default is False.
+        export_formats (list): List of formats that exported at the end of
+            the experiment. Default is None.
         max_failures (int): Try to recover a trial from its last
             checkpoint at least this many times. Only applies if
             checkpointing is enabled. Setting to -1 will lead to infinite
@@ -119,6 +121,7 @@ class Experiment(object):
                  sync_function=None,
                  checkpoint_freq=0,
                  checkpoint_at_end=False,
+                 export_formats=None,
                  max_failures=3,
                  restore=None,
                  repeat=None,
@@ -146,6 +149,7 @@ class Experiment(object):
             "sync_function": sync_function,
             "checkpoint_freq": checkpoint_freq,
             "checkpoint_at_end": checkpoint_at_end,
+            "export_formats": export_formats or [],
             "max_failures": max_failures,
             "restore": restore
         }