Add baseline experiments and associated READMEs

[yining610]

Checkout the baselines directory. All data directories and sub-directories are excluded.

[yining610]

You can check out the repo of the latest commit 71066f9

[yining610]

Please check out src/collate_fns/ecqa_collate_fn.py before writing other ecqa collate function variants to ensure we are using the same data format for this dataset.