Change Grid from ontology to a data structure in core.py

[hepengfe]

This PR fixes #817.

Description of changes

• change Grids -> Grid and move it to core.py as a data structure rather than an ontology
  • dealing edge cases of residual grid cells(last grid cells per row/column). We use the image size as upper bound for these grid cells so they might be smaller than regular grid cells.

Possible influences of this PR.

Describe what are the possible side-effects of the code change.

Test Conducted

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[codecov]

Codecov Report

Merging #876 (9d7824d) into master (12152dc) will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #876      +/-   ##
==========================================
+ Coverage   80.84%   80.85%   +0.01%     
==========================================
  Files         254      254              
  Lines       19444    19460      +16     
==========================================
+ Hits        15719    15734      +15     
- Misses       3725     3726       +1     

Impacted Files	Coverage Δ
forte/data/data_store.py	93.51% <ø> (ø)
forte/data/entry_converter.py	82.89% <ø> (-0.44%)	⬇️
forte/data/ontology/top.py	77.87% <ø> (+<0.01%)	⬆️
forte/data/ontology/core.py	76.77% <76.08%> (-0.15%)	⬇️
forte/data/data_pack.py	85.49% <100.00%> (-0.03%)	⬇️
tests/forte/grid_test.py	100.00% <100.00%> (ø)
forte/pipeline.py	92.53% <0.00%> (+0.21%)	⬆️
forte/data/ontology/ontology_code_const.py	100.00% <0.00%> (+1.53%)	⬆️

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[hunterhector]

Some minor comments on this. Btw, where would the functionality: grid as index for fast overlap computation be implemented? I think that's the actual reason of using grid.