Enabling custom TF signature draft
# What does this PR do?
This PR is related to the discussion on issue #19094 , which is related to `transformers` enabling users to provide custom signatures for models while saving them with [save_pretrained](https://github.com/huggingface/transformers/blob/ca485e562b675341409e3e27724072fb11e10af7/src/transformers/modeling_tf_utils.py#L2085).

Fixes #19094 


## Before submitting
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?
- [x] Did you write any new necessary tests?
_The documentation is not available anymore as the PR was closed or merged._<|||||>Adding a test (perhaps inside [this test class](https://github.com/huggingface/transformers/blob/f3d2f7a6e08efe18debf59512325f02128394b43/tests/test_modeling_tf_common.py#L1913)) would be great!<|||||>Hey there, I think that this code is finished and ready to be merged, I fixed the code and added the tests, let me know if the tests look too hardcoded or if I could check something else.

It seems that the failed tests are not related to my changes.

cc @gante @Rocketknight1 @sgugger <|||||>@dimitreOliveira it seems ready to merge.

There is an apparently unrelated failure, but let's rule it out. Can I request you to rebase and commit? i.e. ensure your fork is up to date with the latest main and then run
```bash
git checkout main
git pull
git checkout custom_tf_signature
git rebase origin/main
git push origin custom_tf_signature -f
```<|||||>@gante should be good now, thanks!