Added encoder argument to BYOL constructor

[garryod]

What does this PR do?

Adds functionality requested in #383

• Added base encoder and MLP dimension (encoder_out_dim, projector_hidden_size & projector_out_dim) arguments to BYOL constructor

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• Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #637 (124fd1b) into master (b087892) will not change coverage.
The diff coverage is 25.00%.

@@           Coverage Diff           @@
##           master     #637   +/-   ##
=======================================
  Coverage   25.54%   25.54%           
=======================================
  Files         118      118           
  Lines        7110     7110           
=======================================
  Hits         1816     1816           
  Misses       5294     5294           

Flag	Coverage Δ
cpu	25.54% <25.00%> (ø)
pytest	25.54% <25.00%> (ø)

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Impacted Files	Coverage Δ
pl_bolts/models/self_supervised/byol/models.py	33.33% <20.00%> (ø)
...l_bolts/models/self_supervised/byol/byol_module.py	22.34% <33.33%> (ø)

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