message: merge tomerk 's proposal
  update document about custom_objects
diff: @@ -95,8 +95,8 @@ def layer_test(layer_cls, kwargs=None, input_shape=None, input_dtype=None,
  string or integer values.
  adapt_data: Optional data for an 'adapt' call. If None, adapt() will not
  be tested for this layer. This is only relevant for PreprocessingLayers.
  - custom_objects: Optional Objects for your custom layer. If you write a
  - your layer, you can use this variable.
  + custom_objects: Optional dictionary mapping name strings to custom objects
  + in the layer class. This is helpful for testing custom layers.
  Returns:
  The output data (Numpy array) returned by the layer, for additional
  