[RLlib] Add tf-graph by default when doing Policy.export_model(). (#7759)

* Rollback.

* WIP.

* WIP.

* Fix.

* LINT.

rllib/policy/tf_policy.py

@@ -347,7 +347,9 @@ class TFPolicy(Policy):
             signature_def_map = self._build_signature_def()
             builder.add_meta_graph_and_variables(
                 self._sess, [tf.saved_model.tag_constants.SERVING],
-                signature_def_map=signature_def_map)
+                signature_def_map=signature_def_map,
+                saver=tf.summary.FileWriter(export_dir).add_graph(
+                    graph=self._sess.graph))
             builder.save()
 
     @override(Policy)

rllib/policy/torch_policy.py

@@ -233,7 +233,6 @@ class TorchPolicy(Policy):
 
     def extra_grad_info(self, train_batch):
         """Return dict of extra grad info."""
-
         return {}
 
     def optimizer(self):