[quant][graphmode] Rename _quantize_script.py to quantize_script.py (#39122)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/39122
Test Plan: Imported from OSS
Differential Revision: D21757619
fbshipit-source-id: 603c020aaaf6f467e63f15b4f271fe946d9fb949
