stop_gradient=True for input spec (#1943)

ppcls/arch/__init__.py

@@ -64,6 +64,7 @@ def apply_to_static(config, model):
         specs = None
         if 'image_shape' in config['Global']:
             specs = [InputSpec([None] + config['Global']['image_shape'])]
+            specs[0].stop_gradient = True
         model = to_static(model, input_spec=specs)
         logger.info("Successfully to apply @to_static with specs: {}".format(
             specs))