quant: rename _ObserverBase to UniformQuantizationObserverBase (#76461)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76461
Renaming as the old name was confusing. The name represents
better what this class is doing.
Test Plan: CI
Reviewed By: jerryzh168
Differential Revision: D35976350
Pulled By: vkuzo
fbshipit-source-id: 6da6c1767cec729c3959b13ae9dd939d0b2f622c
(cherry picked from commit 065608ef42c599525bfad4603af74c5bdf0881c3)
