Prepare io.py for multitask case

[ivyleavedtoadflax]

• Re-applies #22 which was reverted in #23.
• Adds new test for case when empty labels are passed to load_tsv
• Most of the large size comes from a single file containing 1400 rows: tests/prodigy/test_data/test_token_labelled_references.tsv.

[codecov-io]

Codecov Report

Merging #24 into master will decrease coverage by 0.81%. The diff coverage is 86.07%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   84.56%   83.75%   -0.82%     
==========================================
  Files          16       16              
  Lines        1192     1200       +8     
==========================================
- Hits         1008     1005       -3     
- Misses        184      195      +11     

Impacted Files	Coverage Δ
deep_reference_parser/prodigy/prodigy_to_tsv.py	88.23% <83.07%> (-9.25%)	:arrow_down:
deep_reference_parser/io/io.py	97.22% <100.00%> (+0.34%)	:arrow_up:

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