Fix `functional::smooth_l1_loss` signatures to not override `beta` (#109798)
This splits `nn::functional::smooth_l1_loss` into two different signatures in order to keep backward compatibility for calling the function like `smooth_l1_loss(input, target, /*reduction=*/..., /*beta=*/...)`
Fixes #70163
Pull Request resolved: https://github.com/pytorch/pytorch/pull/109798
Approved by: https://github.com/mikaylagawarecki
