Create a symbol function by handle and function name.

def _make_symbol_function(handle, name, func_name):
    """Create a symbol function by handle and function name."""
    code, doc_str = _generate_symbol_function_code(handle, name, func_name)

    local = {}
    exec(code, None, local)  # pylint: disable=exec-used
    symbol_function = local[func_name]
    symbol_function.__name__ = func_name
    symbol_function.__doc__ = doc_str
    symbol_function.__module__ = 'mxnet.symbol'
    return symbol_function