[modelcard] Update dataset tags
Author: sanchit-gandhi
Pull request: yes

# What does this PR do?

Currently, the `model-index` portion of the model cards generated by Trainer reference the train dataset and omit the dataset split and config. This PR:

1. Uses the **eval dataset** to build the yaml data for the model card rather than the **train dataset** by default. Why? Because the yaml data is built on a trio of information of {task, dataset, metrics} (_c.f._ [modelcard.py#L446](https://github.com/huggingface/transformers/blob/d0c1ded5f36e27cd74728c0127add5afdf1f2afa/src/transformers/modelcard.py#L446)). Here, metrics is referring to the **eval dataset** metrics, so we should build the metadata information with the eval dataset name, config, split, etc. If the eval_dataset is None, we revert to the train_dataset.

2. Checks if `dataset_metadata` is None. If so, builds from the `one_dataset`.

The combined changes of 1 and 2 mean that model cards generated by Trainer will be compatible with the autoevaluate leaderboards! https://huggingface.co/spaces/autoevaluate/leaderboards 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] 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.
- [ ] 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?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

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._

Here's an example @lewtun 

Before: https://huggingface.co/sanchit-gandhi/whisper-debug/blob/88128010f73114cc2274868938ccbf6c373b15c5/README.md#L11-L20

After: https://huggingface.co/sanchit-gandhi/whisper-debug/blob/3be9573cff0eb5af8877189481fd13d411171a86/README.md#L9-L20 (only used 8 samples for eval)

Will merge if you're happy with the changes @lewtun?

(merging to unblock testing for the Whisper fine-tuning event)