Add new dataset MMChat
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4423). All of your documentation changes will be reflected on that endpoint. Thanks ! As for https://github.com/huggingface/datasets/pull/4431 please also update the licensing section in https://huggingface.co/datasets/silver/mmchat ;)

Then if it's fine for you feel free to close this PR
Hi, I am adding a new dataset MMChat. 
It seems that all tests are passed