Refactor ArmQuantizer and quantize utils for readabililty (#4095)

Summary:
Add docstrings, type hints and minor fixes that should not change functionality.

Change-Id: Icf0c72f0dbc4a292b605db034aa423a2dc8f3e7b

Pull Request resolved: https://github.com/pytorch/executorch/pull/4095

Reviewed By: mergennachin

Differential Revision: D59259018

Pulled By: digantdesai

fbshipit-source-id: 915b5f53ace2c30226b82ab102e441e24a4fcf43