Update tsv.py

[tonellotto]

The following code doesn't run correctly it the docstore is not already cached in pklz4 folder:

import ir_datasets
import more_itertools

dataset = ir_datasets.load('msmarco-passage')

for batch in more_itertools.chunked(dataset.docs_iter(), 8196*4):
    print(len(batch))

With this single line, it can, without raising exceptions.