[OpenVINO Backend] Support: Implemented numpy.diagonal for keras ov backend

[geeky33]

• 1. Implemented a diagonal function similar to diag, handling 2D+ tensors.
• 2. Extracted diagonals based on the given axis1, axis2, and offset (k).
• 3. Used OpenVINO ops like gather or gather_nd to fetch diagonal elements.

Ticket Id:

#29116

Concerned Authority:

CC: @rkazants

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 6.25000% with 30 lines in your changes missing coverage. Please review.

Project coverage is 82.63%. Comparing base (8333ef4) to head (625445e).

Files with missing lines	Patch %	Lines
keras/src/backend/openvino/numpy.py	6.25%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21086      +/-   ##
==========================================
- Coverage   82.68%   82.63%   -0.05%     
==========================================
  Files         564      564              
  Lines       54114    54144      +30     
  Branches     8410     8415       +5     
==========================================
  Hits        44743    44743              
- Misses       7293     7323      +30     
  Partials     2078     2078              

Flag	Coverage Δ
keras	82.44% <6.25%> (-0.05%)	⬇️
keras-jax	64.00% <0.00%> (-0.04%)	⬇️
keras-numpy	59.09% <0.00%> (-0.04%)	⬇️
keras-openvino	32.84% <6.25%> (-0.02%)	⬇️
keras-tensorflow	64.33% <0.00%> (-0.04%)	⬇️
keras-torch	64.05% <0.00%> (-0.04%)	⬇️

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[rkazants]

switch on the corresponding tests