fix functionalization regression introduced by ProxyTorchDispatchMode, migrate testing to make_fx (#80416)
`ProxyTorchDispatchMode` was added recently as part of `make_fx`, which was secretly causing the meta tensor calls used inside of functionalization to get baked into the graph. It also wasn't caught because the functionalization tests in core don't use `make_fx`, and the tests in functorch aren't as comprehensive.
Now that `make_fx` is in core, I also ported the functionalization test infra over to use it, which would have caught the regression. This also makes the tests cleaner, since mode-based tracing lets us pick up factory functions in the trace output.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80416
Approved by: https://github.com/ezyang, https://github.com/albanD
