Added IS_THREAD_SAFE backend flag

[nicolaspi]

• Added IS_THREAD_SAFE backend flag with an accompanying test.
• Disabled async callbacks for unsafe backends.
• Fixes Low frequency issue using the tensorflow-GPU backend #20382

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.50%. Comparing base (813fbc5) to head (aa31f67).
Report is 11 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (813fbc5) and HEAD (aa31f67). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (813fbc5)	HEAD (aa31f67)
keras	4	3
keras-torch	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20383      +/-   ##
==========================================
- Coverage   78.87%   73.50%   -5.38%     
==========================================
  Files         512      513       +1     
  Lines       49266    49374     +108     
  Branches     7953     7962       +9     
==========================================
- Hits        38861    36294    -2567     
- Misses       8546    11279    +2733     
+ Partials     1859     1801      -58     

Flag	Coverage Δ
keras	73.43% <92.30%> (-5.31%)	⬇️
keras-jax	62.49% <76.92%> (+0.09%)	⬆️
keras-numpy	57.55% <92.30%> (+0.13%)	⬆️
keras-tensorflow	63.64% <76.92%> (+<0.01%)	⬆️
keras-torch	?

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[nicolaspi]

Relevant tensorflow issue: tensorflow/tensorflow#78338

[fchollet]

Neat, thank you for the fix.

[fchollet]

Tests / Check the code format (pull_request) Failing after 2m

Please run sh shell/format.sh