Rename valid_ to val_

[carmocca]

What does this PR do?

Fixes #137

[codecov]

Codecov Report

Merging #197 (fb46458) into master (ba34bf4) will not change coverage.
The diff coverage is 77.77%.

@@           Coverage Diff           @@
##           master     #197   +/-   ##
=======================================
  Coverage   84.30%   84.30%           
=======================================
  Files          55       55           
  Lines        2447     2447           
=======================================
  Hits         2063     2063           
  Misses        384      384           

Flag	Coverage Δ
unittests	84.30% <77.77%> (ø)

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

Impacted Files	Coverage Δ
flash/text/classification/data.py	81.81% <ø> (ø)
flash/text/seq2seq/core/data.py	83.60% <ø> (ø)
flash/text/seq2seq/summarization/data.py	85.71% <ø> (ø)
flash/text/seq2seq/translation/data.py	90.00% <ø> (ø)
flash/vision/embedding/model.py	73.17% <0.00%> (ø)
flash/data/data_module.py	76.76% <58.33%> (ø)
flash/tabular/classification/data/data.py	88.63% <75.00%> (ø)
flash/vision/classification/data.py	87.65% <90.00%> (ø)
flash/data/process.py	77.88% <100.00%> (ø)
flash/vision/classification/model.py	95.45% <100.00%> (ø)
... and 1 more

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