Support output model immediate_ouput

[senwu]

Description of the proposed changes

Support output model immediate_ouput.

Test plan

Added new tests.

Checklist

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

[codecov[bot]]

Codecov Report

Merging #80 (17c3882) into master (d731843) will increase coverage by 0.12%. The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   87.63%   87.75%   +0.12%     
==========================================
  Files          38       38              
  Lines        1714     1740      +26     
  Branches      361      374      +13     
==========================================
+ Hits         1502     1527      +25     
  Misses        113      113              
- Partials       99      100       +1     

Flag	Coverage Δ
unittests	87.75% <96.55%> (+0.12%)	: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	81.59% <96.00%> (+1.21%)	:arrow_up:
src/emmental/task.py	95.45% <100.00%> (+0.71%)	:arrow_up: