Destroy tf.Session when the session is closed.

Previously session destruction was delayed until the destructor for the Python session object.  If the session ends up requiring the Python cycle collector for deallocation, it could end up persisting for a long, non-deterministic period.  This can tie up resources and lead to out of memory issues.

This change introduces a SessionRef which causes session.close() to block until all outstanding run operations are finished and tears down the underlying session.

PiperOrigin-RevId: 205670577