Port `gather` to structured kernel (#63312)
Summary:
Will add a description once this is ready for review.
cc: ysiraichi ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63312
Reviewed By: iramazanli
Differential Revision: D30597447
Pulled By: ezyang
fbshipit-source-id: d36e59835c2f4b38e286032dd2a1111a7e16b7e5
