Support exporting squeeze & unsqueeze with negative dim attribute
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/19297
Reviewed By: zrphercule
Differential Revision: D14953525
Pulled By: houseroad
fbshipit-source-id: 8d7eecd2804b8e27d3ee4ad6e763352818d02d0c
