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

❌ Patch coverage is 0.29940% with 333 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.12%. Comparing base (2eb8e54) to head (f9518a1).
⚠️ Report is 234 commits behind head on main.

Files with missing lines	Patch %	Lines
..._ml/explainer_interfaces/dice_tensorflow2_multi.py	0.00%	331 Missing ⚠️
dice_ml/dice.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #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%)	⬇️

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