fix cyclic imports in self-supervised

[Borda]

What does this PR do?

Fixes #210

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[codecov]

Codecov Report

Merging #350 (a41c3b3) into master (6674ece) will not change coverage.
The diff coverage is 90.00%.

@@           Coverage Diff           @@
##           master     #350   +/-   ##
=======================================
  Coverage   82.37%   82.37%           
=======================================
  Files         100      100           
  Lines        5612     5612           
=======================================
  Hits         4623     4623           
  Misses        989      989           

Flag	Coverage Δ
cpu	24.67% <40.00%> (ø)
pytest	24.67% <40.00%> (ø)
unittests	81.62% <90.00%> (ø)

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Impacted Files	Coverage Δ
pl_bolts/models/self_supervised/resnets.py	92.78% <88.88%> (ø)
pl_bolts/utils/self_supervised.py	100.00% <100.00%> (ø)

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