Simplify a few diagonal-related functions (#87180)
`diag` was unnecessarily implemented as a kernel rather than as a composite
function, which made it unnecessarily difficult (explicit backward + all it entails).
We also change a few uses of `diag` on 2D tensors for `diagonal()`. The
latter returns a view rather than creating a new tensor.
We also upgrade its meta implementation to a fully-fledged
decomposition
I tried implementing the backwards of `diagonal()` via `diag_scatter` (or better `diag_scatter_` to keep the perf) but functionalisation was failing and I was not sure how to fix this, so I moved on. It may be possible to simplify that one as well if @soulitzer or someone knows how to do this.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87180
Approved by: https://github.com/ngimel, https://github.com/albanD, https://github.com/mruberry
