Add pre_metric_transform hook

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #219 (f0faf2b) into master (14502b0) will decrease coverage by 3.70%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   86.56%   82.86%   -3.71%     
==========================================
  Files          57       57              
  Lines        2747     2754       +7     
==========================================
- Hits         2378     2282      -96     
- Misses        369      472     +103     

Flag	Coverage Δ
unittests	82.86% <90.00%> (-3.71%)	⬇️

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

Impacted Files	Coverage Δ
flash/text/classification/model.py	62.50% <0.00%> (-31.25%)	⬇️
flash/core/classification.py	100.00% <100.00%> (ø)
flash/core/model.py	96.31% <100.00%> (+0.09%)	⬆️
flash/tabular/classification/model.py	86.95% <100.00%> (ø)
flash/vision/classification/model.py	95.83% <100.00%> (ø)
flash/text/seq2seq/core/data.py	42.62% <0.00%> (-40.99%)	⬇️
flash/text/classification/data.py	40.90% <0.00%> (-40.91%)	⬇️
flash/text/seq2seq/summarization/data.py	57.14% <0.00%> (-28.58%)	⬇️
flash/text/seq2seq/translation/model.py	55.55% <0.00%> (-16.67%)	⬇️
... and 3 more

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[ethanwharris]

Being dealt with in #216