[ao] quant_type.py fixing public v private (#87519)
Summary: made _get_quant_type_to_str private
Test Plan: python test/test_public_bindings.py
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Differential Revision: [D40709282](https://our.internmc.facebook.com/intern/diff/D40709282)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87519
Approved by: https://github.com/jcaip
