Fix/issue 1032 nan metrics crash

[Quant-Quasar]

Fixes #1032

[gemini-code-assist]

Summary of Changes

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This pull request addresses a critical bug (Issue #1032) in keras_tuner where NaN metric values could lead to a crash or RuntimeWarning during the determination of the best metric value. The changes ensure robust handling of scenarios where all tracked metric observations are NaN, providing a stable and predictable behavior for metric tracking.

Highlights

• NaN Metric Handling: Introduced a pre-check in the get_best_value method to explicitly return np.nan if all observed metric values are NaN. This prevents numpy RuntimeWarning and potential crashes when dealing with entirely undefined metrics.
• Code Reordering: The check for empty _observations was moved to the beginning of the get_best_value method, ensuring an early exit and improved readability.

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[gemini-code-assist]

Code Review

This pull request addresses a crash that occurs when all metric values are NaN. The fix involves adding a check to handle this specific case, preventing a RuntimeWarning from NumPy. The approach is correct and effectively resolves the issue. I have included one suggestion to make the implementation more idiomatic by consistently using NumPy arrays and functions, which could also offer a slight performance improvement.