Fix type conversion in dequant and add a test (#19226)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19226
Type conversoin was wrong previously. Thanks zafartahirov for finding it!
Differential Revision: D14926610
fbshipit-source-id: 6824f9813137a3d171694d743fbb437a663b1f88
