Add in_chans arg to resnet

[ethanwharris]

What does this PR do?

Adds in_chans argument to resnet for consistency with timm

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[codecov]

Codecov Report

Merging #673 (c7b5c2f) into master (e86bfd9) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #673   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files         188      188           
  Lines        9730     9730           
=======================================
  Hits         8782     8782           
  Misses        948      948           

Flag	Coverage Δ
unittests	90.25% <50.00%> (ø)

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

Impacted Files	Coverage Δ
flash/image/classification/backbones/resnet.py	90.85% <50.00%> (ø)

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