Support from_dicts for Tabular Classification and Regression

[krshrimali]

What does this PR do?

Partially fixes #1326

• Tabular Classification and Regression classes now have from_dicts method to load tabular data in the form of dictionaries.
• Tests have been added for both classification and regression.

About the implementation:

• We convert the dictionary to a Pandas DataFrame and then call the method for loading a data frame input.

Docs (for reviewers) artifacts: (note: I just pushed a commit to fix a minor typo, will update these links once CI is done uploading)

• For TabularClassificationData.from_dict
• For TabularRegressionData.from_dict

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

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[codecov]

Codecov Report

Merging #1331 (730f4f2) into master (4a128ce) will increase coverage by 1.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1331      +/-   ##
==========================================
+ Coverage   89.86%   91.45%   +1.59%     
==========================================
  Files         287      287              
  Lines       12900    12921      +21     
==========================================
+ Hits        11592    11817     +225     
+ Misses       1308     1104     -204     

Flag	Coverage Δ
unittests	91.45% <100.00%> (+1.59%)	⬆️

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

Impacted Files	Coverage Δ
flash/tabular/classification/data.py	97.14% <100.00%> (+0.71%)	⬆️
flash/tabular/classification/input.py	96.66% <100.00%> (+0.51%)	⬆️
flash/tabular/regression/data.py	96.77% <100.00%> (+0.77%)	⬆️
flash/tabular/regression/input.py	95.83% <100.00%> (+0.83%)	⬆️
flash/core/serve/dag/task.py	96.82% <0.00%> (-1.06%)	⬇️
flash/text/question_answering/input.py	95.06% <0.00%> (+2.46%)	⬆️
flash/text/seq2seq/translation/model.py	63.63% <0.00%> (+4.54%)	⬆️
flash/text/question_answering/collate.py	95.29% <0.00%> (+4.70%)	⬆️
flash/text/question_answering/model.py	91.15% <0.00%> (+6.12%)	⬆️
flash/text/classification/collate.py	100.00% <0.00%> (+8.33%)	⬆️
... and 15 more

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[ethanwharris]

LGTM 😃 Small suggestion