Data Pipeline V2: Refactor Deserializer into Serve Input

[tchaton]

What does this PR do?

Fixes #916

This change is temporary until we get merge the ServeInput and add serve_load_sample directly to the Input base class.

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[codecov]

Codecov Report

Merging #1013 (d496941) into master (9fac5e2) will increase coverage by 0.05%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master    #1013      +/-   ##
==========================================
+ Coverage   85.28%   85.33%   +0.05%     
==========================================
  Files         254      254              
  Lines       13655    13662       +7     
==========================================
+ Hits        11645    11659      +14     
+ Misses       2010     2003       -7     

Flag	Coverage Δ
unittests	85.33% <96.42%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/data/batch.py	92.10% <ø> (ø)
flash/core/data/io/input_base.py	92.72% <93.33%> (+6.26%)	⬆️
flash/audio/speech_recognition/data.py	73.28% <100.00%> (+0.18%)	⬆️
flash/core/data/data_pipeline.py	96.92% <100.00%> (+0.01%)	⬆️
flash/core/data/process.py	62.06% <100.00%> (-4.60%)	⬇️
flash/core/model.py	86.57% <100.00%> (ø)
flash/image/data.py	82.66% <100.00%> (ø)
flash/image/segmentation/data.py	82.32% <100.00%> (ø)
flash/tabular/data.py	95.10% <100.00%> (ø)
flash/text/classification/data.py	97.01% <100.00%> (ø)
... and 1 more

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[ethanwharris]

Looks good 😃 I guess the existing deserializers should be renamed to end in ServeInput and will also need to extend serve input rather than deserializer