Unsqueeze bug fix (#66889)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66889
Added support for negative dims and modified unit test.
Test Plan: buck test mode/dev-nosan caffe2/test/fx2trt/converters:test_unsqueeze
Reviewed By: 842974287
Differential Revision: D31769393
fbshipit-source-id: 854335ead2ffad5f466ad66b9be36ba20a0fea67
