More forward-over-reverse implementations. (#78740)
Umbrella issue: https://github.com/pytorch/pytorch/issues/75432.
This one implements forward-over-reverse for:
* mse_loss
* l1_loss
* smooth_l1_loss
* softplus
* hardswish (also adds double backward support)
* prelu
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78740
Approved by: https://github.com/soulitzer
