added clip models vocab file to manifest

[maxi-w]

Hey :smile: , when installing lavis via pip the clip vocab file bpe_simple_vocab_16e6.txt.gz is missing and therefore throwing an error FileNotFoundError: [Errno 2] No such file or directory: '/home/maxi/ml/test-lavis/venv/lib/python3.10/site-packages/lavis/models/clip_models/bpe_simple_vocab_16e6.txt.gz'. I added the file to the MANIFEST.in - same as in https://github.com/mlfoundations/open_clip/blob/1c8647f14ff1f826b9096962777e39f7c5cd4ba9/MANIFEST.in - to include it in future releases.