Add configurable metric evaluator to Engine

[giotre]

Modified files:

• src/reax/training/trainer.py

  • Forward metric_evaluator from Trainer to Engine.
  • Update the public metric_evaluator property to return the evaluator owned by Engine.
  • Update eval_stats so it uses the configured engine evaluator by default, while still allowing an explicit per-call override.

• src/reax/_engine.py

  • Add optional metric_evaluator argument to Engine.
  • Store the configured evaluator on the engine.
  • Fall back to DefaultEvaluator when no evaluator is provided.
  • Expose the evaluator through a public metric_evaluator property.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/reax/training/trainer.py	50.00%	1 Missing ⚠️
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Files with missing lines	Coverage Δ
src/reax/_engine.py	85.63% <100.00%> (ø)
src/reax/training/trainer.py	89.73% <50.00%> (ø)

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