Raise an error for is_signed on quantized types (#30527)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30527
When we introduced dtype.is_signed we allowed for support of
quantized types, but we're not sure what the correct result should be.
See discussion at https://github.com/pytorch/pytorch/pull/29511
Test Plan: Imported from OSS
Differential Revision: D18765410
Pulled By: nairbv
fbshipit-source-id: c87cfe999b604cfcbbafa561e04d0d5cdbf41e6d
