feat: add NoImprovementHandler

[Aanushka001]

Description

Adds a NoImprovementHandler that stops training when metrics stop improving for a set number of epochs. This is a simpler alternative to the existing EarlyStopping handler.

Key Features

• Simpler interface: Only requires patience, score_function, trainer, and optional mode parameter
• Mode support: Works with both maximization ('max', default) and minimization ('min') objectives
• Serializable: Supports state_dict() and load_state_dict() for checkpointing
• Resettable: Supports reset() method for reuse across training runs
• Attachable: Standard attach() method for easy integration with ignite engines

Changes

• New file: ignite/handlers/no_improvement_handler.py
• Updated: ignite/handlers/__init__.py to export NoImprovementHandler

Closes #2314

[aaishwarymishra]

Hi, I think the issue meant this handler should take a custom function or method and execute it in case of no improvement. Currently the implementation looks a loot like early stopping instead of a modular handler.

[Aanushka001]

Thanks for the feedback! I've updated the PR to add a `custom_action` parameter. Users can now pass any callable to execute on no improvement (e.g., reducing LR, checkpointing, logging, or terminating) instead of being restricted to just trainer termination. The trainer parameter is now optional when custom_action is provided. Here's the commit: Aanushka001@9da7116 Would appreciate a re-review when you get a chance!

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