[quant][pt2e] change internal code to only import from _quantize_pt2e (#105162)
Summary: This is to make public api clear so that we can make implementation details change easier in the future
Test Plan: CIs
Differential Revision: D47445767
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105162
Approved by: https://github.com/andrewor14
