[fx2trt]Add support for negative dim in softmax (#66760)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66760
Previously we didn't convert negative dim to positive dim.
Test Plan: WIP
Reviewed By: wushirong
Differential Revision: D31703127
fbshipit-source-id: 6d5ccecab45b46f867a05ee70c76a5980e41011d
