Add support for conditional detr
Author: DeppMeng
Pull request: yes

# What does this PR do?

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Added codes and documentations for conditioonal DETR model. The conditional DETR files are created by using the "add-new-model-like" feature of CookieCutter, based on DETR codes. All tests are passed. One thing I want to ask is that, I have converted the pretrained weights, how shoud I give these weights to you?


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case. https://github.com/Atten4Vis/ConditionalDETR/issues/21
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

The CI issue is caused by the fact that you have the following lines in src/transformers/models/auto/feature_extraction_auto.py:

```
("detr", "DetrFeatureExtractor"),
("detr", "DetrFeatureExtractor"),
```

=> this should be updated to:

```
("detr", "DetrFeatureExtractor"),
("conditional_detr", "ConditionalDetrFeatureExtractor"),
```

Thanks a lot for all your work 🤗 merging!