[Hook] Add 'before_create_session' interface to SessionRunHook. (#991)

Signed-off-by: chenbangduo.cbd <[email protected]>

Author: JackMoriarty

tensorflow/python/training/monitored_session.py

@@ -957,6 +957,8 @@ def __init__(self, session_creator, hooks, stop_grace_period_secs):
     def create_session(self):
       """Creates a coordinated session."""
       # Keep the tf_sess for unit testing.
+      for hook in self._hooks:
+        hook.before_create_session()
       self.tf_sess = self._session_creator.create_session()
       # We don't want coordinator to suppress any exception.
       self.coord = coordinator.Coordinator(clean_stop_exception_types=[])
@@ -1027,6 +1029,7 @@ class MonitoredSession(_MonitoredSession):
   in given order:
 
   * calls `hook.begin()` for each given hook
+  * calls `hook.before_create_session()`
   * finalizes the graph via `scaffold.finalize()`
   * create session
   * initializes the model via initialization ops provided by `Scaffold`

tensorflow/python/training/session_run_hook.py

@@ -109,6 +109,20 @@ def begin(self):
     """
     pass
 
+  def before_create_session(self):
+    """Called before new TensorFlow session is created.
+
+    This has two essential differences with the situation in which `begin` is
+    called:
+
+    * Do not modify the graph in this method, ops should not be added to graph.
+        The modification of the graph should take place within the begin
+        interface.
+    * This method will also be called prior to the recovery of a wrapped
+        session, not just at the beginning of the overall session.
+    """
+    pass
+
   def after_create_session(self, session, coord):  # pylint: disable=unused-argument
     """Called when new TensorFlow session is created.