Tabular regression task and example

[Borda]

What does this PR do?

Fixes #905

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[codecov]

Codecov Report

Merging #892 (c58ef25) into master (d0adc61) will decrease coverage by 0.14%.
The diff coverage is 60.93%.

@@            Coverage Diff             @@
##           master     #892      +/-   ##
==========================================
- Coverage   88.47%   88.33%   -0.15%     
==========================================
  Files         239      241       +2     
  Lines       12961    13017      +56     
==========================================
+ Hits        11467    11498      +31     
- Misses       1494     1519      +25     

Flag	Coverage Δ
unittests	88.33% <60.93%> (-0.15%)	⬇️

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

Impacted Files	Coverage Δ
flash/tabular/regression/model.py	51.35% <51.35%> (ø)
flash/core/model.py	88.05% <66.66%> (-0.15%)	⬇️
flash/core/regression.py	66.66% <66.66%> (ø)
flash/core/classification.py	94.15% <100.00%> (+0.03%)	⬆️
flash/tabular/__init__.py	100.00% <100.00%> (ø)
flash/tabular/classification/model.py	92.50% <100.00%> (-0.53%)	⬇️
flash/tabular/regression/__init__.py	100.00% <100.00%> (ø)

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[ethanwharris]

Awesome, example looks great 😃 Mind adding some docs / tests? (can be in a follow-up 😃)