Make parameter value more generic and adaptable.

Incorporated the suggested changes.

research/object_detection/exporter.py

@@ -69,7 +69,7 @@ def freeze_graph_with_def_protos(
     if optimize_graph:
       logging.info('Graph Rewriter optimizations enabled')
       rewrite_options = rewriter_config_pb2.RewriterConfig(
-          layout_optimizer=1)
+          layout_optimizer=rewriter_config_pb2.RewriterConfig.ON)
       rewrite_options.optimizers.append('pruning')
       rewrite_options.optimizers.append('constfold')
       rewrite_options.optimizers.append('layout')