Kill unused _pointwise_loss function. (#43523)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43523
The code is also wrong, see https://github.com/pytorch/pytorch/issues/43228.
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D23305461
Pulled By: gchanan
fbshipit-source-id: 9fe516d87a4243d5ce3c29e8822417709a1d6346
