change pre_autograd to pre_dispatch tracing (#101818)
We discussed in a composability meeting a few weeks ago that `pre_autograd` should probably be renamed to `pre_dispatch`.
One question in this PR was: should I re-use a dispatch key? Or should I create a new dispatch key (that yet again corresponds to "top of the dispatcher")?
~~For now, I ended up sticking our proxy mode on the mode stack corresponding to `PythonTLSSnapshot`, because it was simple and it works. It looks like one of the functorch dispatch keys has higher priority though, so it's possible that functorch will end up running first. Open to options, but we can consider adding a new dispatch key later if that becomes a problem~~
Update: I added a dedicated dispatch key, `PreDispatch`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101818
Approved by: https://github.com/ezyang, https://github.com/Neilblaze, https://github.com/albanD, https://github.com/zou3519
