bugfix: batch_size for MNISTDataModule

[hecoding]

What does this PR do?

Properly fixes #171 fixes #334
batch_size wasn't getting saved at the constructor and thus defaulting to 32. Now it behaves like other datamodules (i.e. CIFAR10DataModule).

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[codecov]

Codecov Report

Merging #331 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
+ Coverage   82.04%   82.20%   +0.15%     
==========================================
  Files          98       98              
  Lines        5492     5501       +9     
==========================================
+ Hits         4506     4522      +16     
+ Misses        986      979       -7     

Flag	Coverage Δ
cpu	24.26% <ø> (+0.01%)	⬆️
pytest	24.26% <ø> (+0.01%)	⬆️
unittests	81.58% <ø> (+0.15%)	⬆️

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

Impacted Files	Coverage Δ
...l_bolts/models/self_supervised/swav/swav_module.py	73.30% <0.00%> (-1.05%)	⬇️
pl_bolts/datamodules/__init__.py	88.67% <0.00%> (+0.67%)	⬆️
...l_bolts/models/rl/vanilla_policy_gradient_model.py	94.64% <0.00%> (+2.67%)	⬆️
pl_bolts/optimizers/lars_scheduling.py	95.74% <0.00%> (+17.02%)	⬆️

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[Borda]

lgtm