Adding optional sampler input to DataModule

[lillekemiker]

What does this PR do?

This PR adds the option of specifying a sampler to be used in the training DataLoader returned by a DataModule.

Fixes # 350

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[lillekemiker]

Could someone supply me with info on how I should add this to the CHANGELOG.md? Considering that it technically changes the user facing API, I guess the semantic version should be bumped on the "minor" level, but that just seems big for this pretty small change.

[codecov]

Codecov Report

Merging #390 (a4dbbc2) into master (b49bf04) will decrease coverage by 6.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #390      +/-   ##
==========================================
- Coverage   89.79%   83.40%   -6.39%     
==========================================
  Files          80       80              
  Lines        4203     4207       +4     
==========================================
- Hits         3774     3509     -265     
- Misses        429      698     +269     

Flag	Coverage Δ
unittests	83.40% <100.00%> (-6.39%)	⬇️

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	93.05% <100.00%> (-0.35%)	⬇️
flash/text/seq2seq/summarization/metric.py	26.98% <0.00%> (-68.26%)	⬇️
flash/text/classification/data.py	36.49% <0.00%> (-51.10%)	⬇️
flash/text/seq2seq/core/data.py	37.33% <0.00%> (-50.01%)	⬇️
flash/text/classification/model.py	40.00% <0.00%> (-50.00%)	⬇️
flash/core/schedulers.py	55.55% <0.00%> (-44.45%)	⬇️
flash/text/seq2seq/core/model.py	41.42% <0.00%> (-31.43%)	⬇️
flash/text/seq2seq/summarization/model.py	57.89% <0.00%> (-21.06%)	⬇️
flash/text/seq2seq/translation/model.py	55.00% <0.00%> (-20.00%)	⬇️
flash/text/seq2seq/summarization/utils.py	50.00% <0.00%> (-20.00%)	⬇️
... and 5 more

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[ethanwharris]

@lillekemiker Awesome! LGTM. Regarding the CHANGELOG, could you just add a line under Added in the Unreleased section at the top, no need for a version bump 😃

[edgarriba]

LGTM. Just small suggestion