dbr quant: remove more asserts from hot paths (#68431)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68431
asserts have some overhead, removing the asserts used only to make
mypy happy from the path which is hit in every forward.
Test Plan: python test/test_quantization.py TestQuantizeDBR
Reviewed By: jerryzh168
Differential Revision: D32463767
Pulled By: vkuzo
fbshipit-source-id: 5f85f80144f35a725afe481bf027ea61ca6315bf
