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

def replace_variable_node(node, annotation):
    """Replace a node annotated by `nni.variable`.
    node: the AST node to replace
    annotation: annotation string
    """
    assert type(node) is ast.Assign, 'nni.variable is not annotating assignment expression'
    assert len(node.targets) == 1, 'Annotated assignment has more than one left-hand value'
    name, expr = parse_nni_variable(annotation)
    assert test_variable_equal(node.targets[0], name), 'Annotated variable has wrong name'
    node.value = expr
    return node