Enhance the robustness of the flash attention check

[james77777778]

Also fixes GPU CI.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.06%. Comparing base (0861e09) to head (94f25ce).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/src/backend/torch/nn.py	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20495      +/-   ##
==========================================
- Coverage   82.09%   82.06%   -0.03%     
==========================================
  Files         515      515              
  Lines       47575    47615      +40     
  Branches     7463     8531    +1068     
==========================================
+ Hits        39056    39077      +21     
- Misses       6710     6721      +11     
- Partials     1809     1817       +8     

Flag	Coverage Δ
keras	81.92% <82.35%> (-0.03%)	⬇️
keras-jax	64.99% <35.29%> (-0.03%)	⬇️
keras-numpy	59.95% <11.76%> (-0.03%)	⬇️
keras-tensorflow	65.98% <11.76%> (-0.03%)	⬇️
keras-torch	64.91% <58.82%> (-0.02%)	⬇️

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[james77777778]

This PR should be ready to run on GPU CI again now. I’m wondering if it’s possible to use a newer GPU for Keras GPU CI, as flash attention isn't available on T4, and these tests are currently being skipped.

Colab: https://colab.research.google.com/drive/1-fQdyAs-w5lM7ZGN8mroWmQP9HBJxYcK?usp=sharing

EDIT:
This PR pins coverage==7.6.1 to overcome this issue: nedbat/coveragepy#1891

BTW, I saw the announcement. Good luck and best wishes, @fchollet!
I've really enjoyed collaborating with you, and I’m proud of my contributions to Keras. Looking forward to seeing your next move!

[fchollet]

LGTM, tank you!

[nedbat]

The assert in coverage.py 7.6.5 is now fixed and released as part of coverage 7.6.6.