Variable used only in type annotations are not captured to the transformed functions

[Shizhi Tang]

Example:

def main():

    import freetensor as ft

    # ERROR: `ft` not found
    @ft.transform(verbose=2)
    def test(x: ft.Var[(4,), "float32"]):
        y = x + 1
        return y

    # OK
    #@ft.transform(verbose=2)
    #def test(x: ft.Var[(4,), "float32"]):
    #    y = ft.zeros((4,), "float32")
    #    y[...] = x + 1
    #    return y


main()

ft is captured if it is used in ft.zeros, but it is not captured if only used in ft.Var.