Hard error when ignoring tensors. (#27484)
Author: Narsil
Pull request: yes

* [WIP] Hard error when ignoring tensors.

* Better selection/error when saving a checkpoint.

- Find all names we should normally drop (those are in the transformers
  config)
- Find all disjoint tensors (for those we can safely trigger a copy to
  get rid of the sharing before saving)
- Clone those disjoint tensors getting rid of the issue
- Find all identical names (those should be declared in the config
  but we try to find them all anyway.)
- For all identical names:
  - If they are in the config, just ignore them everything is fine
  - If they are not, warn about them.
- For all remainder tensors which are shared yet neither identical NOR
  disjoint. raise a hard error.

* Adding a failing test on `main` that passes here.

* We don't need to keep the subfolder logic in this test.

* Apply suggestions from code review

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

---------

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

Should fix #29903, fixes #28293
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_29906). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

> Only concern is that we are still vulnerable to _tied_weights being modified after instance creation, but I don't really see an easy way to prevent this other than giving warnings here.

Those are private therefore it should be OK. You can make it immutable through @property but that seems a bit too much at this point.