Update parallel.py to use root_device instead of lightning_module.device

[ananthsub]

What does this PR do?

Minor refactor to use the strategy's own root_device instead of the LightningModule's device property.

Attempts at manual model parallelization by extending this plugin will face difficulties with the assumption that the LightningModule has all of its parameters on the same device.

For those use cases, it is critical to remove the assumption that the module has a device property (device in general goes against PyTorch module's design principles:

• [Feature Request] nn.Module should also get a device attribute pytorch/pytorch#7460
• Device #1790 (comment)

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

No

Before submitting

• [n/a] Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• [n/a] Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

cc @Borda @awaelchli @rohitgr7 @akihironitta @justusschock