Add eli5_category dataset
This pull request adds a categorized Long-form question answering dataset `ELI5_Category`. It's a new variant of the [ELI5](https://huggingface.co/datasets/eli5) dataset that uses the Reddit tags to alleviate the training/validation overlapping in the origin ELI5 dataset.

A [report](https://celeritasml.netlify.app/posts/2021-12-01-eli5c/)(Section 2) on this dataset. 
> Thanks a lot for adding this dataset ! Good job with the dataset card and the dataset scripts - they're really good :)
> 
> I just added minor changes

Thanks for fixing typos!