Fix NoneType issue in collate_fn when using python 3.8+ on mac

[senwu]

Description of the problems or issues

On Mac with python 3.8+ multiprocessing uses spawn to start multiple process and it won't inherit from the origin process which causes Meta.config is None. This results emmental_collate_fn cannot get min_data_len and max_data_len from Meta.config.

Description of the proposed changes

Instead get min_data_len and max_data_len from Meta.config in Emmental_collate_fn, we set those two variables in the EmmentalDataLoader.

Test plan

Passes the existing test.

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 #117 (b417339) into master (17b5075) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage   92.12%   92.13%   +0.01%     
==========================================
  Files          40       40              
  Lines        2018     2021       +3     
  Branches      431      432       +1     
==========================================
+ Hits         1859     1862       +3     
  Misses         94       94              
  Partials       65       65              

Flag	Coverage Δ
unittests	92.13% <100.00%> (+0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
src/emmental/data.py	91.74% <100.00%> (+0.23%)	:arrow_up: