add an env var for dispatcher debug logging (#81846)
Partially addresses https://github.com/pytorch/pytorch/issues/81703
Committing a patch @zou3519 came up with a while ago - with a `DEBUG=1` build, you can now use `SHOW_DISPATCH_TRACE=1 python script.py` to get chatty logs of all of the dispatcher calls that happened in a script.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81846
Approved by: https://github.com/ezyang, https://github.com/zou3519
