Add support for (input, target) style datasets (e.g. torchvision) to from_datasets

[ethanwharris]

What does this PR do?

Fixes #373

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[tchaton]

Neat !

[ananyahjha93]

is there a reason for using 196 as h, w for imagenet? I thought the default was 224.

[codecov]

Codecov Report

Merging #552 (f463cab) into master (5d98cde) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #552   +/-   ##
=======================================
  Coverage   91.30%   91.31%           
=======================================
  Files         112      112           
  Lines        7132     7127    -5     
=======================================
- Hits         6512     6508    -4     
+ Misses        620      619    -1     

Flag	Coverage Δ
unittests	91.31% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/data/data_source.py	94.31% <100.00%> (+0.33%)	⬆️

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