Use root_device in DeviceStatsMonitor callback

[ananthsub]

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

Attempts at manual model parallelization will face difficulties with the assumption that the LightningModule has a well-defined device property. Not all of the model parameters will be on the same device.

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

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

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