[fx2trt]fix slice tensor converter (#65960)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65960
Fix a bug in the converter and add support for negative dim.
Test Plan: buck test mode/dev-nosan caffe2/torch/fb/fx2trt:test_narrow
Reviewed By: wushirong
Differential Revision: D31310232
fbshipit-source-id: 62887369d830202cae6d63b41747225b12dcf754
