Add warning to set_backend and more detailed example.

[hertschuh]

Addressing comments from #21764

[gemini-code-assist]

Summary of Changes

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This pull request improves the user experience and safety when using the set_backend utility in Keras. It introduces a prominent warning to alert users to the potential pitfalls of switching backends dynamically, such as issues with existing Keras objects. Additionally, it provides a more comprehensive and illustrative example, along with a docstring clarification, to guide users through the correct procedure for changing backends and ensuring all related modules are properly reloaded.

Highlights

• Warning Added to set_backend: A UserWarning has been integrated into the keras.config.set_backend function to explicitly caution users about the implications of dynamically changing the Keras backend, particularly regarding already-instantiated objects and the necessity of re-importing modules.
• Enhanced Example for set_backend: The example for keras.config.set_backend has been significantly expanded to provide a more detailed and practical demonstration of switching backends, including setting environment variables, verifying tensor types, and the critical step of re-importing Keras modules after a backend change.
• Docstring Clarification: The docstring for set_backend has been updated to clarify that all imported Keras modules need to be re-imported, not just the keras module itself, to ensure the backend change is fully applied.

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[hertschuh]

@yufengg

[gemini-code-assist]

Code Review

This pull request improves the set_backend function by adding a runtime warning and a more detailed usage example in the docstring. These changes significantly enhance the user experience by making the function's behavior and potential risks much clearer. The new example is particularly helpful. I've identified a minor grammatical error in the updated docstring and have provided a suggestion for a fix.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.63%. Comparing base (eecd34f) to head (b155895).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/src/utils/backend_utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21787      +/-   ##
==========================================
- Coverage   82.63%   82.63%   -0.01%     
==========================================
  Files         577      577              
  Lines       59316    59318       +2     
  Branches     9300     9300              
==========================================
+ Hits        49018    49019       +1     
- Misses       7910     7911       +1     
  Partials     2388     2388              

Flag	Coverage Δ
keras	82.46% <50.00%> (-0.01%)	⬇️
keras-jax	63.32% <50.00%> (-0.01%)	⬇️
keras-numpy	57.55% <50.00%> (-0.01%)	⬇️
keras-openvino	34.29% <50.00%> (+<0.01%)	⬆️
keras-tensorflow	64.11% <50.00%> (-0.01%)	⬇️
keras-torch	63.62% <50.00%> (-0.01%)	⬇️

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