Use L1 loss for Smooth L1 loss with beta=0 (#97022)
Fixes #96813.
Comments:
1. Wasn't able to test since tools/nightly.py does not allow for GPU build (and I don't want to build from scratch).
2. In theory, the bug (i.e. NaNs) can still occur when beta is very small (e.g. `beta=1e-50`), but not sure whether anybody cares.
3. Some checks within the smooth_l1_loss C++ code could be changed to check for `beta > 0` instead of `beta >= 0`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97022
Approved by: https://github.com/jbschlosser
