Add Table Transformer
# What does this PR do?

This PR adds [Table Transformer](https://github.com/microsoft/table-transformer) by Microsoft, which are DETR-compatible models for table detection and table structure recognition tasks in unstructured documents.

Note: I'm making some updates to the original DETR implementation, however these are justified by the fact that the original DETR implementation by Facebook AI also includes these things, which I didn't add when first porting DETR. Hence, our DETR implementation is now more aligned with the original one.

To do:

- [ ] transfer checkpoints to the Microsoft organization
- [ ] add link to notebook


_The documentation is not available anymore as the PR was closed or merged._<|||||>I'm very much not in favor of adding a new config parameter that controls where the layernorm is applied. I'm not surprised the original code has it, as Facebook AI usually codes models in a modular way, but not Transformers. We had the same thing with BART and friends, and they are coded as distinct models in the library.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Closing this PR in favor of #19614