Add has_registered_events method to Engine class

[JeevanChevula]

Adds a new method has_registered_events(event) -> bool to the Engine class to check whether a specific event is registered with the engine.

This method will be used by the Checkpoint handler to determine if custom events need to be registered, improving separation of concerns between Engine and Checkpoint classes.

Related to PR #3440 (checkpoint events feature)

[vfdev-5]

Thanks for the quick PR @JeevanChevula
Please also write tests with built-in events in test_engine.py and custom events in test_custom_events.py

[vfdev-5]

LGTM, thanks @JeevanChevula !
Let's see if the CI is passing (we can have some unrelated failure on the CI)

[vfdev-5]

@JeevanChevula can you please code styling issue: https://github.com/pytorch/ignite/actions/runs/17579433225/job/49932341505?pr=3449

[JeevanChevula]

Fixed formatting issues

[JeevanChevula]

I investigated the CI failures and found the error message 'Please remove this workaround and restore dataset check.' appearing in all failed runs. I'm unsure of the exact cause, but I believe this may be an error independent of the PR functionality, as it appears to be related to Caltech101 dataset handling rather than the Engine method implementation