Add Serving to RunningStage

[tchaton]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #872 (d1e0237) into master (b22e786) will decrease coverage by 0.04%.
The diff coverage is 85.24%.

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
- Coverage   85.05%   85.00%   -0.05%     
==========================================
  Files         227      228       +1     
  Lines       12600    12632      +32     
==========================================
+ Hits        10717    10738      +21     
- Misses       1883     1894      +11     

Flag	Coverage Δ
unittests	85.00% <85.24%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/utilities/stages.py	73.91% <73.91%> (ø)
flash/core/data/properties.py	94.11% <81.25%> (-5.89%)	⬇️
flash/core/data/auto_dataset.py	100.00% <100.00%> (ø)
flash/core/data/base_viz.py	100.00% <100.00%> (ø)
flash/core/data/batch.py	95.93% <100.00%> (ø)
flash/core/data/callback.py	100.00% <100.00%> (ø)
flash/core/data/data_module.py	94.18% <100.00%> (ø)
flash/core/data/data_pipeline.py	95.23% <100.00%> (-0.02%)	⬇️
flash/core/data/data_source.py	94.78% <100.00%> (ø)
flash/core/data/datasets.py	94.17% <100.00%> (ø)
... and 13 more

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