Add property getter for modules

[zcbenz]

We need a way to lazily initialized module attributes, i.e. def_prop for modules, there are 2 use cases in MLX:

• Avoid initializing mx.random.state at import time (to avoid initializing GPU too early).
• Make mx.random.state a thread-local variable (to support multi-threads).

[wjakob]

It sounds interesting! Would you be willing to contribute such a feature?

[zcbenz]

I did some research on this and found that python actually has a feature exactly for this purpose, what I asked can be done with:

  m.def("__getattr__", [&](nb::handle key) -> nb::object {
    if (nb::isinstance<nb::str>(key) && nb::cast<std::string>(key) == "state") {
      return state;
    }
    return nb::steal(PyErr_Format(
        PyExc_AttributeError,
        "Module 'random' has no attribute %R",
        key.ptr()));
  });