Fix _module_available to detect horovod.torch properly

[JoostvDoorn]

What does this PR do?

Fixes #12370, such that horovod.torch is again properly detected.

This PR simplifies the logic of the _module_available function. I think in general it's not a good idea to try to deviate too much from importlib when testing for module availability, if the specific logic of hasattr is necessary for any check it would be better to handle that case separately. I tried reproducing the error that first triggered this change in the torchmetrics repo Lightning-AI/torchmetrics#770 by testing _module_available("transformers.models.auto") and this didn't give me any problems, but a maintainer might be a better judge of this. I can replicate this code to that repo later.

Note I run horovod in develop mode not sure if that's related, but latest pypi version of horovod has a very restrictive pin on the pytorch lightning version.

Does your PR introduce any breaking changes? If yes, please list them.

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[awaelchli]

Thanks for the PR @JoostvDoorn
I triggered CI, let's see what happens :)

[JoostvDoorn]

Some of the horovod tests are failing and seem to be skipped in master.

[Borda]

does it also work for any other modules?
we had the for loop there on purpose...

[JoostvDoorn]

does it also work for any other modules? we had the for loop there on purpose...

I tested some manually, but I think this is a good point, so I added some tests in 0d79821 to confirm imports match with the flags allowing the CI to test for this. Let me know if there are any specific cases that need to be added to this, as the exact flags for which the loop is relevant in this repository isn't fully clear to me.

[awaelchli]

@JoostvDoorn It looks like #12379 will have to be completed first, right? If you want, you can merge #12379 into this branch to see if both work together.

[awaelchli]

Great!