Split workload when use multiprocessing for loading data.

[Albert-Ma]

#150

[codecov]

Codecov Report

Merging #151 (0e5c04e) into dev (0f08a4f) will decrease coverage by 54.39%.
The diff coverage is 0.00%.

@@ Coverage Diff @@ ## dev #151 +/- ## =========================================== - Coverage 95.01% 40.61% -54.40%  =========================================== Files 104 104 Lines 3811 3811 =========================================== - Hits 3621 1548 -2073  - Misses 190 2263 +2073 

Impacted Files	Coverage Δ
matchzoo/dataloader/callbacks/padding.py	13.11% <0.00%> (-84.43%)	⬇️
matchzoo/models/bimpm.py	13.97% <0.00%> (-86.03%)	⬇️
matchzoo/modules/stacked_brnn.py	20.00% <0.00%> (-80.00%)	⬇️
matchzoo/modules/semantic_composite.py	22.22% <0.00%> (-77.78%)	⬇️
matchzoo/dataloader/callbacks/ngram.py	15.55% <0.00%> (-77.78%)	⬇️
matchzoo/modules/spatial_gru.py	20.75% <0.00%> (-77.36%)	⬇️
matchzoo/modules/matching.py	23.07% <0.00%> (-76.93%)	⬇️
matchzoo/models/esim.py	23.43% <0.00%> (-76.57%)	⬇️
matchzoo/preprocessors/basic_preprocessor.py	24.07% <0.00%> (-75.93%)	⬇️
... and 80 more

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