fix imports

[Borda]

What does this PR do?

Fixes #239
Split SSL callback into two files so they can be imported

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[codecov]

Codecov Report

Merging #288 into master will increase coverage by 0.06%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
+ Coverage   81.85%   81.91%   +0.06%     
==========================================
  Files          98       99       +1     
  Lines        5495     5497       +2     
==========================================
+ Hits         4498     4503       +5     
+ Misses        997      994       -3     

Flag	Coverage Δ
#cpu	23.49% <44.82%> (+<0.01%)	⬆️
#pytest	23.49% <44.82%> (+<0.01%)	⬆️
#unittests	81.35% <96.55%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
pl_bolts/models/self_supervised/cpc/cpc_module.py	20.31% <0.00%> (-0.79%)	⬇️
pl_bolts/callbacks/byol_updates.py	100.00% <100.00%> (ø)
pl_bolts/callbacks/ssl_online.py	100.00% <100.00%> (ø)
...l_bolts/models/self_supervised/byol/byol_module.py	84.94% <100.00%> (ø)
...lts/models/self_supervised/simclr/simclr_module.py	82.81% <100.00%> (ø)
...ts/models/self_supervised/swav/swav_online_eval.py	22.95% <100.00%> (+1.63%)	⬆️
...l_bolts/models/rl/vanilla_policy_gradient_model.py	94.64% <0.00%> (+2.67%)	⬆️

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