Replace a node annotated by `nni.function_choice`.
    node: the AST node to replace
    annotation: annotation string

def replace_function_node(node, annotation):
    """Replace a node annotated by `nni.function_choice`.
    node: the AST node to replace
    annotation: annotation string
    """
    target, funcs = parse_nni_function(annotation)
    FuncReplacer(funcs, target).visit(node)
    return node