force task_to_label_dict to be a dict

[senwu]

Description of the problems or issues

Is your pull request related to a problem? Please describe. There is unclear part about task_to_label_dict which can be a dict or a list.

Description of the proposed changes

Force task_to_label_dict to be a dict.

Test plan

Add new e2e test to verify this.

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 #106 (24c781e) into master (d6e0342) will increase coverage by 0.77%. The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   85.00%   85.77%   +0.77%     
==========================================
  Files          41       41              
  Lines        2000     2010      +10     
  Branches      428      433       +5     
==========================================
+ Hits         1700     1724      +24     
+ Misses        175      171       -4     
+ Partials      125      115      -10     

Flag	Coverage Δ
unittests	85.77% <77.77%> (+0.77%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/emmental/model.py	81.47% <72.00%> (+0.86%)	:arrow_up:
src/emmental/data.py	91.50% <90.90%> (+5.66%)	:arrow_up:
src/emmental/learner.py	69.96% <0.00%> (+0.99%)	:arrow_up:
src/emmental/task.py	87.50% <0.00%> (+3.12%)	:arrow_up:
src/emmental/scorer.py	80.00% <0.00%> (+7.50%)	:arrow_up: