Why there is a `device_id` parameter in `nn.module.cpu()` call?

`pytorch/torch/nn/modules/modules.py`
```python
def cpu(self, device_id=None):
    """Moves all model parameters and buffers to the CPU."""
    return self._apply(lambda t: t.cpu())
```

as shown in source code, this parameters is not used. Any reason that we keep it?