Feature/gelu shift negative

[yarden-yagil-sony]

Pull Request Description:

Add GELU activation operator to shift negative substitustions in both keras and pytorch.

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).

[Idan-BenAmi]

General comment: check if you need to modify the mct tests

[ofirgo]

@yardeny-sony, please add a description to the PR, explaining the main changes and possible design choices that were made (if there are such)