[functorch] linearize (#94173)
Fixes https://github.com/pytorch/functorch/issues/724
TODO:
* [x] Docs
NOTE: `const_fold` pass raises UserWarning -> https://github.com/pytorch/pytorch/issues/94374
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94173
Approved by: https://github.com/Chillee
