adding TabFact dataset
Adding TabFact: A Large-scale Dataset for Table-based Fact Verification.

https://github.com/wenhuchen/Table-Fact-Checking

- The tables are stored as individual csv files, so need to download 16,573 🤯  csv files. As a result the `datasets_infos.json` file is huge (6.62 MB).
- Original dataset has nested structure where, where table is one example and each table has multiple statements,
flattening the structure here so that each statement is one example.