Convert padding layer from keras to coreml.
    Keras only supports zero padding at this time.
    Parameters
    ----------
    keras_layer: layer
        A keras layer object.

    builder: NeuralNetworkBuilder
        A neural network builder object.

def convert_cropping(builder, layer, input_names, output_names, keras_layer):
    """Convert padding layer from keras to coreml.
    Keras only supports zero padding at this time.
    Parameters
    ----------
    keras_layer: layer
        A keras layer object.

    builder: NeuralNetworkBuilder
        A neural network builder object.
    """
    # Get input and output names
    input_name, output_name = (input_names[0], output_names[0])

    if isinstance(keras_layer, keras.layers.convolutional.Cropping1D):
        left, right = keras_layer.cropping
        top, bottom = (0, 0)
    else: # 2D
        left, right = keras_layer.cropping[0]
        top, bottom = keras_layer.cropping[1]

    # Now add the layer
    builder.add_crop(name = layer,
        left = left, right=right, top=top, bottom=bottom, offset = [0,0],
        input_names = [input_name], output_name=output_name
        )