[ao] fuser_method_mappings.py fixing public v private (#87516)
Summary: made _get_valid_patterns, _DEFAULT_PATTERN_TO_FUSER_METHOD,
_reverse3, _reverse2, _reverse_sequential_wrapper2,
_DEFAULT_OP_LIST_TO_FUSER_METHOD, _sequential_wrapper2 private
Test Plan: python test/test_public_bindings.py
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Differential Revision: [D40709281](https://our.internmc.facebook.com/intern/diff/D40709281)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87516
Approved by: https://github.com/jcaip
