Adding max_iters as an optional arg in Engine run

[thescripted]

Fixes #1300

Description:
New tests have not yet been written, and I believe some descriptions/comments will need to be updated. I'm happy to do that, but I'm opening up this PR for feedback and general indications to see if I'm on the right track.

I've assumed that reaching max iteration will fire a "Terminate" event rather than "Epoch Completed," but I don't know if either of those events are accurate for this condition.

FInally, adding a condition to this block:

ignite/ignite/engine/engine.py

Lines 557 to 558 in a8a9c7c

	def _is_done(state: State) -> bool:
	return state.iteration == state.epoch_length * state.max_epochs

currently breaks some tests, so I have left that alone for the moment.

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)

[sdesrozis]

@thescripted Thank you ! I left few comments.

[vfdev-5]

@thescripted Thanks for working on the PR !
I left few comment on how to improve it

[vfdev-5]

@thescripted could you also please resolve the conflict and update your branch to master.

[thescripted]

On it. Sorry for the late reply I've put a bit on my plate with other projects. I'll go ahead and resolve these issues now.

[vfdev-5]

Thanks @thescripted ! Looks good

I still have few comments about the PR. For example, a special care is needed here

while self.state.epoch < self.state.max_epochs

if max_epochs is None, right ?

[thescripted]

Yes @vfdev-5, I didn't consider that. I believe a fix could be like so:

while not self.should_terminate:  # type: ignore[operator]
    if self.state.max_epochs is not None and self.state.epoch >= self.state.max_epochs:
        break

I also think a condition to check the max_iters here is required.

ignite/ignite/engine/engine.py

Lines 559 to 560 in acd2982

	def _is_done(state: State) -> bool:
	return state.iteration == state.epoch_length * state.max_epochs # type: ignore[operator]

From what I can tell max_epochs will never be none, given where and how the method is used Here and Here. I can just add another conditional checking if state.iterations == state.max_iters which may or may not be None.

If those are the right track (and any additional comments you have) I'll be happy to fix those up.

[vfdev-5]

@thescripted good point about _is_done. So, I'm thinking that we can replace the code

while self.state.epoch < self.state.max_epochs and not self.should_terminate:

by

def _is_done(state: State) -> bool: 
     # something like that
     c1 = state.max_iters and state.iterations == state.max_iters
     c2 = state.epoch_length and state.iteration == state.epoch_length * state.max_epochs     
     return c1 or c2

while not self._is_done(self.state) and not self.should_terminate:

What do you think ?