Change references to migrated variable_scope functions.

PiperOrigin-RevId: 528182254

Author: fionalang

tensorflow_estimator/python/estimator/tpu/tpu_estimator.py

@@ -43,6 +43,7 @@
 from tensorflow.python.framework import ops
 from tensorflow.python.ops import control_flow_ops
 from tensorflow.python.ops import control_flow_util
+from tensorflow.python.ops import ref_variable
 from tensorflow.python.ops import summary_ops_v2
 from tensorflow.python.ops import variable_scope
 from tensorflow.python.platform import tf_logging as logging
@@ -115,8 +116,8 @@
   ops.register_proto_function(
       '{}_{}'.format(_TPU_ESTIMATOR, _ITERATIONS_PER_LOOP_VAR),
       proto_type=variable_pb2.VariableDef,
-      to_proto=variable_scope._to_proto_fn,  # pylint: disable=protected-access
-      from_proto=variable_scope._from_proto_fn)  # pylint: disable=protected-access
+      to_proto=ref_variable._to_proto_fn,  # pylint: disable=protected-access
+      from_proto=ref_variable._from_proto_fn)  # pylint: disable=protected-access
 
 
 def _is_iterable(obj):