Rank Zero Deprecation

[lcaquot94]

I get this warning when launching training with RayStrategy on my pl.LightningModule on multiples workers :
LightningDeprecationWarning: pytorch_lightning.utilities.warnings.rank_zero_deprecation has been deprecated in v1.6 and will be removed in v1.8. Use the equivalent function from the pytorch_lightning.utilities.rank_zero module instead.

[amogkam]

Hey @lcaquot94, thanks for reporting this! Indeed, we should change this- a PR would be much appreciated!