At end of epoch on an IterableDataset ensure engine.state.output rema…

[mtauraso]

…ins.

Fixes #3372

Description: Implements the fix described in the bugreport

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)

[vfdev-5]

Thanks for the PR @mtauraso , I have another suggestion here to approximatively keep the previous behavior.

[vfdev-5]

@mtauraso can you please run code formatting script: bash ./tests/run_code_style.sh install && bash ./tests/run_code_style.sh fmt to fix:

+ flake8 ignite tests examples --config setup.cfg
ignite/engine/engine.py:1084:88: W291 trailing whitespace
ignite/engine/engine.py:1271:88: W291 trailing whitespace
ignite/engine/engine.py:1274:1: W293 blank line contains whitespace

[mtauraso]

Ran the formatter and committed. My apologies for the churn, I see now this is all documented in the first time contributors guide which I didn't read earlier.

[vfdev-5]

Thanks for the updates and no worries about the contributing guidelines. By the way, I almost forgot about that we also need to add a test for the feature we adding. Can you please create a test here checking that state.output is not None for the iterable input data, below this line:

ignite/tests/ignite/engine/test_engine.py

Line 1051 in f4f7d1c

assert evaluator.state.iteration == size

Maybe something like this:

def test_iterator_state_output(self):
    ...

you can take a look for inspiration this test: test_faq_inf_iterator_with_epoch_length

[mtauraso]

I found another bug while testing this 🙃

If there's an obvious fix I'd like to know, and would be happy for someone else to move the diff forward, but I probably won't have time to look in detail for a couple days.

If I find a fix I'll add it to the branch and ping again.

[vfdev-5]

@mtauraso thanks for adding the tests. Let me reproduce the bug and make a separate ticket for it. This PR looks good to me and solves the initial problem. Let's land it and maybe work on the other bug in separate PR.

[vfdev-5]

LGTM, thanks @mtauraso !