Abort `reload()` if receiving another `reload()` call

[Neet-Nestor]

Currently, if we call engine.reload() while a loading is already in progress, the ongoing one will not be aborted but instead two reload will run concurrently and the callback will be called for all these progresses. This is not the preferred behavior.

Instead, it's better to cancel or abort the previous reload before initializing the new model reloading. This behavior is intuitive and easier to manage.