[inductor][ez] add template/externchoice uid (#161341)

# why

- to have a central registry of templates/externkernelchoice
  to match them to heuristics etc, they need unique names
- mm is both the triton template name and the aten_mm name

# what

- add a uid() to KernelTemplate/ExternKernelChoice that returns name
- override in ExternKernel to prepend "aten::"
- override in TritonTemplate to prepend "triton::"

This id is just use to find template heuristics, so it has no other
impact

# testing

```
python3 -bb -m pytest test/inductor/test_max_autotune.py -v
```

Differential Revision: [D81520579](https://our.internmc.facebook.com/intern/diff/D81520579)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/161341
Approved by: https://github.com/jansel, https://github.com/eellison
ghstack dependencies: #162075, #161340

torch/_inductor/codegen/common.py

@@ -2391,6 +2391,17 @@ class KernelTemplate:
     def __init__(self, name: str) -> None:
         self.name = name
 
+    @property
+    def uid(self) -> str:
+        """
+        entry point to override for templates to ensure a uid e.g. through a prefix
+
+        the purpose of this is that every KernelTemplate/ExternKernelChoice is unique
+        in the system, but reproducible e.g. restarting pytorch should yield the same id
+        """
+        # TODO(coconutruben): add some central registration to assert on global uniqueness
+        return self.name
+
     def maybe_append_choice(
         self, choices: list[Any], **kwargs: Any
     ) -> Optional[NotImplementedError]: