Fix json serialization in Lambda layer (#3012)

Fix #2582
Fix #3001

keras/layers/core.py

@@ -460,9 +460,9 @@ def get_config(self):
 
         if isinstance(self._output_shape, python_types.LambdaType):
             if py3:
-                output_shape = marshal.dumps(self._output_shape.__code__)
+                output_shape = marshal.dumps(self._output_shape.__code__).decode('raw_unicode_escape')
             else:
-                output_shape = marshal.dumps(self._output_shape.func_code)
+                output_shape = marshal.dumps(self._output_shape.func_code).decode('raw_unicode_escape')
             output_shape_type = 'lambda'
         elif callable(self._output_shape):
             output_shape = self._output_shape.__name__
@@ -494,7 +494,7 @@ def from_config(cls, config):
         if output_shape_type == 'function':
             output_shape = globals()[config['output_shape']]
         elif output_shape_type == 'lambda':
-            output_shape = marshal.loads(config['output_shape'])
+            output_shape = marshal.loads(config['output_shape'].encode('raw_unicode_escape'))
             output_shape = python_types.FunctionType(output_shape, globals())
         else:
             output_shape = config['output_shape']