Create copy of parameters

def copy_param(exe, new_param=None):
    """Create copy of parameters"""
    if new_param is None:
        new_param = {k: nd.empty(v.shape, ctx=mx.cpu()) for k, v in exe.arg_dict.items()}
    for k, v in new_param.items():
        exe.arg_dict[k].copyto(v)
    return new_param