[OpenVINO Backend] Support np.diag

[srinjoydutta03]

• Implemented np.diag for openvino backend
• Closes #29115

CC: @rkazants

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Codecov Report

Attention: Patch coverage is 73.33333% with 12 lines in your changes missing coverage. Please review.

Project coverage is 82.43%. Comparing base (da10cf3) to head (a2911f5).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/src/backend/openvino/numpy.py	73.33%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20967   +/-   ##
=======================================
  Coverage   82.42%   82.43%           
=======================================
  Files         561      561           
  Lines       53226    53247   +21     
  Branches     8244     8250    +6     
=======================================
+ Hits        43871    43892   +21     
- Misses       7338     7344    +6     
+ Partials     2017     2011    -6     

Flag	Coverage Δ
keras	82.24% <73.33%> (+0.01%)	⬆️
keras-jax	63.98% <0.00%> (-0.02%)	⬇️
keras-numpy	58.80% <0.00%> (-0.01%)	⬇️
keras-openvino	32.65% <73.33%> (+0.05%)	⬆️
keras-tensorflow	64.42% <0.00%> (-0.05%)	⬇️
keras-torch	64.04% <0.00%> (-0.01%)	⬇️

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[srinjoydutta03]

I'm not sure why tests for diagflat and diagonal are getting invoked, when I only removed diag from exclude_concrete_tests.txt file. Please help me resolve this.

[srinjoydutta03]

@rkazants Please review whenever you can can. Thank you