[Task]: Implement backwards messages in `Gamma` node

[wouterwln]

Parent Feature

#552

Task Summary

GammaShapeScale has messages towards a and b. This means that the backwards messages for the Gamma node also exist, but they are not implemented for this node.

Task Description

Implement rules for a and theta in Gamma node.

Acceptance Criteria

• Rule towards a is tested and implemented
• Rule towards theta is tested and implemented

Implementation Notes

No response

Dependencies

No response

Priority

Low

Estimated Effort

2 hours

Related Issues / PRs

No response

Additional Context

No response