Gd multi class

[ah-ansari]

This is an extension of the Gradient-based method to handle the multi class case (for tensorflow2 only).

The method is added as a separate class, DiceTensorFlow2Multi.
It can be used by specifying method="multi":
exp = dice_ml.Dice(d, m, method="multi")

[codecov-commenter]

Codecov Report

Merging #309 (f9518a1) into master (2eb8e54) will decrease coverage by 6.25%.
The diff coverage is 0.29%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   71.37%   65.12%   -6.26%     
==========================================
  Files          26       27       +1     
  Lines        3462     3796     +334     
==========================================
+ Hits         2471     2472       +1     
- Misses        991     1324     +333     

Flag	Coverage Δ
unittests	65.12% <0.29%> (-6.26%)	⬇️

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

Impacted Files	Coverage Δ
..._ml/explainer_interfaces/dice_tensorflow2_multi.py	0.00% <0.00%> (ø)
dice_ml/dice.py	79.06% <33.33%> (-3.44%)	⬇️

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