Add COCO dataset
The CI failures other than a missing dummy data file and missing fields in the card are unrelated to this PR.  Thanks a lot for this great work and fixing TFDS based script @mariosasko 🤗 will generate the dummy dataset and write the model card tomorrow! @mariosasko I added the dataset card, I'm on the dummy data rn.  @merveenoyan Let me know if you need any help with the dummy data.

I plan to split the current script/dataset into 4 smaller scripts/datasets to make sure they are properly indexed by Papers With Code later on. In this format:
* the `*_image_captioning` configs will form the [COCO Captions](https://paperswithcode.com/sota/image-captioning-on-coco-captions) dataset (also present in TFDS, but only the 2017 version)
* the `stuff_segmentation` config will form the [COCO Stuff](https://paperswithcode.com/dataset/coco-stuff) dataset
* the `desnepose` config will form the [DensePose-COCO](https://paperswithcode.com/dataset/densepose) dataset
* the rest will be [COCO](https://paperswithcode.com/dataset/coco) (+ will add the `minival` and the `valminusminival` splits to COCO 2014)

Also, if I find the time, I'll add preprocessing examples that rely on `pycocotools` to the README files. @mariosasko I feel like we can just push main COCO and add Captions + Stuff later, WDYT?
This PR adds the MS COCO dataset. Compared to the [TFDS](https://github.com/tensorflow/datasets/blob/master/tensorflow_datasets/object_detection/coco.py) script, this implementation adds 8 additional configs to cover the tasks other than object detection.

Some notes:
* the data exposed by TFDS is contained in the `2014`, `2015`, `2017` and `2017_panoptic_segmentation` configs here
* I've updated `encode_nested_example` for easier handling of missing values (cc @lhoestq @albertvillanova; will add tests if you are OK with the changes in `features.py`)
* this implementation should fix https://github.com/huggingface/datasets/pull/3377#issuecomment-985559427

TODOs:
- [x] dataset card
- [ ] dummy data

cc @merveenoyan 

Closes #2526 