Replace GpuStatsMonitor with DeviceStatsMonitor and accelerator with strategy

Got a deprecation warning:

LightningDeprecationWarning: The `GPUStatsMonitor` callback was deprecated in v1.5 and will be removed in v1.7.
Please use the `DeviceStatsMonitor` callback instead.

as well as:

LightningDeprecationWarning: Passing `Trainer(accelerator='ddp')` has been deprecated in v1.5 and will be removed in v1.7.
Use `Trainer(strategy='ddp')` instead.