Rename DataSource to Input

[ananyahjha93]

What does this PR do?

Fixes #915
Fixes #921

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[codecov]

Codecov Report

Merging #929 (9c8b70f) into master (00ef240) will decrease coverage by 5.18%.
The diff coverage is 84.28%.

@@            Coverage Diff             @@
##           master     #929      +/-   ##
==========================================
- Coverage   84.81%   79.62%   -5.19%     
==========================================
  Files         246      248       +2     
  Lines       12982    13019      +37     
==========================================
- Hits        11011    10367     -644     
- Misses       1971     2652     +681     

Flag	Coverage Δ
unittests	79.62% <84.28%> (-5.19%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/data/callback.py	100.00% <ø> (ø)
flash/core/data/new_data_module.py	88.09% <ø> (ø)
flash/tabular/__init__.py	100.00% <ø> (ø)
flash/text/seq2seq/core/model.py	45.12% <0.00%> (-21.96%)	⬇️
flash/pointcloud/detection/open3d_ml/app.py	26.59% <16.66%> (ø)
flash/image/face_detection/model.py	39.28% <20.00%> (ø)
flash/tabular/regression/model.py	51.35% <20.00%> (ø)
flash/image/face_detection/data.py	43.66% <23.07%> (ø)
flash/text/question_answering/model.py	26.89% <25.00%> (-1.38%)	⬇️
flash/audio/classification/transforms.py	86.95% <33.33%> (ø)
... and 99 more

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[ethanwharris]

Looks good, I think we should try to deprecate DefaultDataKeys if possible

[tchaton]

LGTM ! After all conflicts.