Replaced _lstsq_helper with internal dispatch (#54724)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54724
Removed at::_lstsq_helper; it is replaced with DEFINE/DECLARE_DISPATCH.
Test Plan: Imported from OSS
Reviewed By: H-Huang
Differential Revision: D27993747
Pulled By: mruberry
fbshipit-source-id: dc8b884fd33b3dd18d9a8e4c582b869ac5391de5
