[ao] quantize.py fixing public v private (#87521)
Summary: made _register_activation_post_process_hook, _add_observer,
_get_unique_devices_, _get_observer_dict private
Test Plan: python test/test_public_bindings.py
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Differential Revision: [D40709277](https://our.internmc.facebook.com/intern/diff/D40709277)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87521
Approved by: https://github.com/jerryzh168
