Image getter setter

[hepengfe]

This PR fixes https://github.com/asyml/forte/issues/873.

Description of changes

• DataPack.set_image() that takes image data and set the cache data of image payload at the specified index.
• DataPack.image that getting image data from ImagePayload

Possible influences of this PR.

• it's more convenient for users to get and set image data in data pack.

Test Conducted

• test setting index beyond datapack payloads length
• test get image from datapack

[codecov[bot]]

Codecov Report

Merging #891 (c24dcdd) into master (c22fecb) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #891      +/-   ##
==========================================
+ Coverage   80.88%   80.91%   +0.03%     
==========================================
  Files         254      254              
  Lines       19525    19551      +26     
==========================================
+ Hits        15792    15819      +27     
+ Misses       3733     3732       -1     

Impacted Files	Coverage Δ
forte/data/ontology/top.py	78.17% <ø> (+0.07%)	:arrow_up:
forte/data/data_pack.py	85.88% <100.00%> (+0.39%)	:arrow_up:
tests/forte/image_annotation_test.py	100.00% <100.00%> (ø)

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