[upstream_ut] test_grad_scaler_deprecated_warning_xpu: wrong deprecation message (cpu instead of xpu)

## Description
`test_grad_scaler_deprecated_warning_xpu` expects a `torch.xpu.amp.GradScaler` deprecation warning but the emitted warning refers to `torch.cpu.amp.GradScaler`.

## Error log
```
AssertionError: "`torch.xpu.amp.GradScaler\(args...\)` is deprecated." does not match "`torch.cpu.amp.GradScaler(args...)` is deprecated. Please use `torch.amp.GradScaler('cpu', args...)` instead."
```

## Reproduce
Using pytorch/pytorch#196421 on a PVC machine:
```
python test/test_torch.py TestTorchDeviceTypeXPU.test_grad_scaler_deprecated_warning_xpu
```

## Expected
torch.xpu.amp.GradScaler emits an xpu-specific deprecation message referencing torch.amp.GradScaler('xpu', ...).

_Filed with AI assistance; reproduced per the details above._

Run test/test_torch.py with TestTorchDeviceTypeXPU.test_grad_scaler_deprecated_warning_xpu to reproduce the mismatch. Trace the deprecation warning for torch.xpu.amp.GradScaler and update it so the message references torch.amp.GradScaler('xpu', ...), then rerun the named test to confirm the expected wording.

Written by the indexing model from the issue text.