message: Add lookup table size ops to checkpointing whitelist.
  Reading the size doesn't actually alter the state of the lookup table,
  so it's safe to checkpoint.
diff: @@ -140,11 +140,13 @@ REGISTER_OP("LookupTableSize")
  .Input("table_handle: Ref(string)")
  .Output("size: int64")
  .SetShapeFn(TwoElementVectorInputsAndScalarOutputs);
  +WHITELIST_STATEFUL_OP_FOR_DATASET_FUNCTIONS("LookupTableSize");
  REGISTER_OP("LookupTableSizeV2")
  .Input("table_handle: resource")
  .Output("size: int64")
  .SetShapeFn(ScalarAndTwoElementVectorInputsAndScalarOutputs);
  +WHITELIST_STATEFUL_OP_FOR_DATASET_FUNCTIONS("LookupTableSizeV2");
  REGISTER_OP("LookupTableExport")
  .Input("table_handle: Ref(string)")
  