Fix sampler instance support

[ethanwharris]

What does this PR do?

Fixes #1197

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[Borda]

so this is the alternative for CrossEntropyLoss weights?

[codecov]

Codecov Report

Merging #1204 (a3eda82) into master (8abc10a) will decrease coverage by 8.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1204      +/-   ##
==========================================
- Coverage   90.96%   82.74%   -8.22%     
==========================================
  Files         284      284              
  Lines       12689    12691       +2     
==========================================
- Hits        11542    10501    -1041     
- Misses       1147     2190    +1043     

Flag	Coverage Δ
unittests	82.74% <100.00%> (-8.22%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/data/data_module.py	93.79% <100.00%> (-0.31%)	⬇️
flash/core/integrations/icevision/transforms.py	18.24% <0.00%> (-75.00%)	⬇️
...lash/image/embedding/vissl/transforms/multicrop.py	27.27% <0.00%> (-69.70%)	⬇️
flash/core/integrations/icevision/data.py	28.88% <0.00%> (-66.67%)	⬇️
flash/image/embedding/heads/vissl_heads.py	36.36% <0.00%> (-63.64%)	⬇️
flash/image/detection/input.py	36.66% <0.00%> (-63.34%)	⬇️
...ash/image/classification/integrations/baal/loop.py	32.08% <0.00%> (-62.69%)	⬇️
flash/core/integrations/icevision/backbones.py	33.33% <0.00%> (-60.01%)	⬇️
...ash/image/classification/integrations/baal/data.py	34.34% <0.00%> (-59.60%)	⬇️
...age/face_detection/backbones/fastface_backbones.py	42.10% <0.00%> (-57.90%)	⬇️
... and 47 more

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[krshrimali]

LGTM, thanks! Just minor nit (whitespace).