[Frontend] Refactor call tree verification for Grad

[dime10]

The primitives for Function and Grad have been changed to bind the callee, instead of the object instance itself.
Additionally, the call tree traversal of the callee has been improved to be more robust.

No (external) functional changes.

[erick-xanadu]

🐞

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (de8a59f) 99.50% compared to head (59f1607) 99.48%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
frontend/catalyst/pennylane_extensions.py	90.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #399      +/-   ##
==========================================
- Coverage   99.50%   99.48%   -0.02%     
==========================================
  Files          42       42              
  Lines        7048     7053       +5     
  Branches      476      477       +1     
==========================================
+ Hits         7013     7017       +4     
  Misses         20       20              
- Partials       15       16       +1     

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