def _get_input(layer, index=0):
    """
    :param layer : NN Layer Proto message
    :param index : Layer input index (Default 0)
    :returns name of input at provided index if present, otherwise None
    """
    if len(layer.input) <= index:
        return None
    return layer.input[index]

    :param layer : NN Layer Proto message
    :param index : Layer input index (Default 0)
    :returns name of input at provided index if present, otherwise None
    
coremltools/converters/mil/backend/nn/passes/mlmodel_passes.py