Add complex support for torch.nn.L1Loss (#49912)
Summary:
Building on top of the work of anjali411 (https://github.com/pytorch/pytorch/issues/46640)
Things added in this PR:
1. Modify backward and double-backward formulas
2. Add complex support for `new module tests` and criterion tests (and add complex tests for L1)
3. Modify some existing tests to support complex
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49912
Reviewed By: zhangguanheng66
Differential Revision: D25853036
Pulled By: soulitzer
fbshipit-source-id: df619f1b71c450ab2818eb17804e0c55990aa8ad
