remove Path.exists from bigbio to support streaming

[galtay]

makes the changes from @albertvillanova 's PR https://github.com/bigscience-workshop/biomedical/pull/754/files

tl;dr this fixes a problem in bigbiohub.parse_brat_file that prevented using it in streaming mode. see

• https://huggingface.co/datasets/bigbio/chia/discussions/4

for more information. this new version of bigbiohub.py has been uploaded to all hub datasets.