Update engine.cpp (#120773)
Minor comment fix; `backward` and `grad` are flipped here. See https://pytorch.org/docs/stable/_modules/torch/autograd.html#backward
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120773
Approved by: https://github.com/albanD, https://github.com/janeyx99, https://github.com/soulitzer
