Remove active argument in loss function and ignore_index argument

[senwu]

Description of the problems or issues

Is your pull request related to a problem? Please describe. The argument active usage in learning is not very clear and can be applied directly in loss function.

Description of the proposed changes

Remove active in learning and ignore_index argument in configuration.

Test plan

Update the e2e tests.

Checklist

• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] I have updated the CHANGELOG.rst accordingly.

[codecov[bot]]

Codecov Report

Merging #107 (98e0bc0) into master (2c27905) will increase coverage by 0.42%. The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   86.80%   87.22%   +0.42%     
==========================================
  Files          41       41              
  Lines        2031     2004      -27     
  Branches      438      425      -13     
==========================================
- Hits         1763     1748      -15     
+ Misses        158      153       -5     
+ Partials      110      103       -7     

Flag	Coverage Δ
unittests	87.22% <94.11%> (+0.42%)	:arrow_up:

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Impacted Files	Coverage Δ
src/emmental/scorer.py	80.00% <ø> (ø)
src/emmental/task.py	87.50% <ø> (ø)
src/emmental/utils/parse_args.py	99.32% <ø> (-0.01%)	:arrow_down:
src/emmental/model.py	83.75% <92.85%> (+2.28%)	:arrow_up:
src/emmental/data.py	91.50% <100.00%> (ø)
src/emmental/modules/embedding_module.py	98.24% <100.00%> (ø)