docs: add warmup scheduler mixed-events example

[nightcityblade]

Fixes #2441

Description:

Adds a docstring example showing how to combine Ignite event filters so warm-up runs on iteration starts and the wrapped scheduler runs on later epoch starts.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[TahaZahid05]

Suggested change

	event_filter=lambda engine, event: engine.state.iteration <= 5
	event_filter=lambda engine, event: event <= 5

We already have the count inside event

[TahaZahid05]

@vfdev-5 This PR solves the documentation issue. However, I think I agree with #2441 pushback on maybe this needs an API fix. Currently, we are teaching the user to do the math on when to switch. It would be better if this logic can be internal to improve user experience.

Something like:

scheduler = create_lr_scheduler_with_warmup(
    torch_lr_scheduler,
    warmup_duration=5,
    warmup_event=Events.ITERATION_STARTED,
    post_warmup_event=Events.EPOCH_STARTED
)
# The scheduler would internally handle the event-switching logic.
trainer.add_event_handler(Events.ITERATION_STARTED | Events.EPOCH_STARTED, scheduler)

[nightcityblade]

Thanks for calling that out. I agree the more ergonomic long-term answer is probably an API improvement so users do not need to manually compute the switch point. I kept this PR narrowly scoped to the current docs issue because it only documents the behavior that exists today, but I’m happy for maintainers to treat the API direction separately if that’s preferred.