Create Softmax layer in activation decomposition instead of Activation layer

[reuvenperetz]

Pull Request Description:

In MP we need to know the axis used in Softmax for KL-Divergence computation. In activation decomposition, we created a simple 'Activation' layer thus we can not use the axis info when computing MP distance. This commit changes the decomposition to create a Softmax layer, thus enabling the KL-Divergence in MP distance computation.

Checklist before requesting a review:

• I set the appropriate labels on the pull request.
• I have added/updated the release note draft (if necessary).
• I have updated the documentation to reflect my changes (if necessary).
• All function and files are well documented.
• All function and classes have type hints.
• There is a licenses in all file.
• The function and variable names are informative.
• I have checked for code duplications.
• I have added new unittest (if necessary).