Payload data setter

[hepengfe]

This PR fixes #890

Description of changes

• support datapack.set_text("text", -1) which is equivalent to append text payload
• support datapack.set_text("text") which the text payload index is by default 0. When there is no text payloads in the datapack, it will add a new payload.

Possible influences of this PR.

• user can set/add data of modality in the DataPack easily

Test Conducted

• it's backward compatible with past tests

[codecov]

Codecov Report

Merging #892 (d48b1a9) into master (d5e69a4) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #892      +/-   ##
==========================================
- Coverage   80.95%   80.95%   -0.01%     
==========================================
  Files         254      254              
  Lines       19569    19566       -3     
==========================================
- Hits        15843    15840       -3     
  Misses       3726     3726              

Impacted Files	Coverage Δ
forte/data/data_pack.py	85.90% <ø> (ø)
tests/forte/grid_test.py	100.00% <100.00%> (ø)
tests/forte/image_annotation_test.py	100.00% <100.00%> (ø)

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[hunterhector]

interesting idea, could consider improving the docstring, or maybe re-think the behavior so it won't make people confused.