[mta] Backward of unary foreach functions (#89591)
as per title, this PR defines backward of those.
This doesn't implement forward-mode automatic differentiation as [the current codegen](https://github.com/pytorch/pytorch/blob/a747326423ed4731996769e3b8eb73eecbdee2d4/tools/autograd/gen_variable_type.py#L1513) doesn't seem to handle `ArrayRef<Tensor>`.
Rel:
- https://github.com/pytorch/pytorch/issues/53796
- https://github.com/pytorch/pytorch/issues/58833
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89591
Approved by: https://github.com/albanD
