[doc] expose torch.Tensor.xpu API to doc (pytorch#126383)

# Motivation
The doc string related `torch.Tensor.xpu` has been added [here](https://github.com/pytorch/pytorch/blob/d61a81a9e76688ac8f338a6cfba932bf7779e5ce/torch/_tensor_docs.py#L1434) but not expose it to public doc, like [torch.Tensor.cuda](https://pytorch.org/docs/stable/generated/torch.Tensor.cuda.html#torch.Tensor.cuda). This PR intends to expose the document of `torch.Tensor.xpu` to public doc.

Pull Request resolved: pytorch#126383
Approved by: https://github.com/albanD

tools/pyi/gen_pyi.py

@@ -1130,6 +1130,18 @@ def replace_special_case(hint: str) -> str:
                     )
                 )
             ],
+            "xpu": [
+                "def xpu({}) -> Tensor: ...".format(
+                    ", ".join(
+                        [
+                            "self",
+                            "device: Optional[Union[_device, _int, str]] = None",
+                            "non_blocking: _bool = False",
+                            "memory_format: torch.memory_format = torch.preserve_format",
+                        ]
+                    )
+                )
+            ],
             "cpu": [
                 "def cpu(self, memory_format: torch.memory_format = torch.preserve_format) -> Tensor: ..."
             ],