[fx2trt] add unsqueeze converter (#63355)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63355
Added converter for acc_ops.unsqueeze. Needed for ig model.
DIdn't add support for input that has more than one dynamic dim. This is not needed right now and I feel it would be a rare case.
Test Plan: unit test
Reviewed By: yinghai
Differential Revision: D30138293
fbshipit-source-id: 899fe8eb68387de83195a2f6e199618d96f09a9e
