fix typing

[senwu]

Description of the problems or issues

There are some typing issue with new mypy.

Test plan

Pass mypy tests.

Checklist

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

[codecov[bot]]

Codecov Report

Merging #97 (ef172fe) into master (627ec39) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #97   +/-   ##
=======================================
  Coverage   85.99%   85.99%           
=======================================
  Files          39       39           
  Lines        1892     1892           
  Branches      406      406           
=======================================
  Hits         1627     1627           
  Misses        153      153           
  Partials      112      112           

Flag	Coverage Δ
unittests	85.99% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/emmental/learner.py	70.27% <100.00%> (ø)