Revert deprecation of device flags

[rohitgr7]

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Fixes #<issue_number>

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cc @tchaton @rohitgr7 @akihironitta @justusschock @kaushikb11 @awaelchli @Borda @ananthsub @ninginthecloud @jjenniferdai

[daniellepintz]

why are we reverting this? Could you please add some details to the description?

[awaelchli]

@rohitgr7 In our brief discussion earlier with Will and team we were just talking about the trainer arguments. The properties can be deprecated more freely as the impact and number of code changes on the user side is rare and small. So I don't think this PR is necessary.

[carmocca]

I don't think these should be un-deprecated.

The deprecation concerns only the Trainer's properties but does not touch the __init__'s arguments of the same name.

The suggestion of using num_devices is natural, especially after the accelerator connector rewrite.

[awaelchli]

If we could just revert this addition here, then that should be all I think. Couldn't find any others. We can simply resume efforts on #11040 after the release 😃

edit: done in #12495 (comment)

[kaushikb11]

The deprecation of the Trainer properties was also a natural progression toward deprecating the Trainer flags. As we are moving towards the accelerator agnostic attributes for the users.

IMO, if we are not deprecating Trainer(gpus=x) in this release and extending it to the next major release, we should do the same for Trainer.gpus, etc. Users wouldn't understand the reason why we are deprecating gpus, tpu_cores, etc and moving towards num_devices unless they are enforced to use the devices=x logic.

[DuYicong515]

Users wouldn't understand the reason why we are deprecating gpus, tpu_cores, etc and moving towards num_devices unless they are enforced to use the devices=x logic.

Currently, tpu_cores, num_processes, ipus return number of devices, and gpus return the trainer argument of gpus/devices or what's decided by auto etc. I feel it's a bit confusing when the properties have the same name with the arguments, but return values are different from the argument, and are not consistent across the 4 properties.

I think suggesting user to use device agnostic properties to access their device count or ids is natural even we don't deprecate the arguments. The suggested properties are more general and have clear expectations on the return values from their names. In addition, if we decide they'll be removed eventually, I feel it's better to add deprecation message and prepare people early.

[daniellepintz]

Why not just do the deprecation of the constructor arguments in this release as well? #11040 is ready to go

[carmocca]

We are not comfortable doing this very impactful change for the community one day away from the release.

[awaelchli]

I'm closing this, as 1.6 has now been released. Happy deleting code everybody!