[docs] Fix ScalarTensor __repr__ in Extending PyTorch example (#86330)

This PR fixes the __repr__ of the `ScalarTensor` class in the Extending PyTorch example to correspond with the class name instead of `DiagonalTensor`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86330
Approved by: https://github.com/bdhirsh
diff --git a/docs/source/notes/extending.rst b/docs/source/notes/extending.rst
index 645d687..f7cc413 100644
--- a/docs/source/notes/extending.rst
+++ b/docs/source/notes/extending.rst
@@ -372,7 +372,7 @@
             self._value = value
 
         def __repr__(self):
-            return "DiagonalTensor(N={}, value={})".format(self._N, self._value)
+            return "ScalarTensor(N={}, value={})".format(self._N, self._value)
 
         def tensor(self):
             return self._value * torch.eye(self._N)
@@ -409,7 +409,7 @@
           self._value = value
 
       def __repr__(self):
-          return "DiagonalTensor(N={}, value={})".format(self._N, self._value)
+          return "ScalarTensor(N={}, value={})".format(self._N, self._value)
 
       def tensor(self):
           return self._value * torch.eye(self._N)
@@ -494,7 +494,7 @@
 
   >>> s = ScalarTensor(2, 2)
   >>> torch.add(s, s)
-  DiagonalTensor(N=2, value=4)
+  ScalarTensor(N=2, value=4)
   >>> t = torch.tensor([[1, 1,], [1, 1]])
   >>> torch.add(s, t)
   tensor([[3., 1.],