Support integral target for the backward of nn.SmoothL1Loss (#61112)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/58816
- enhance the backward of `nn.SmoothL1Loss` to allow integral `target`
- add test cases in `test_nn.py` to check the `input.grad` between the integral input and its floating counterpart.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61112
Reviewed By: mrshenli
Differential Revision: D29775660
Pulled By: albanD
fbshipit-source-id: 544eabb6ce1ea13e1e79f8f18c70f148e92be508
