[quant] Use PlaceholderObserver as default dynamic quant observer (#45343)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45343
Current default dynamic quant observer is not correct since we don't accumulate
min/max and we don't need to calculate qparams.
Test Plan: Imported from OSS
Reviewed By: supriyar
Differential Revision: D23933995
fbshipit-source-id: 3ff497c9f5f74c687e8e343ab9948d05ccbba09b
