Add functions from torch.masked._ops to __all__ for torch.masked (#131288)

Add the non-private operations imported in this file to `__all__` so that pyright considers them to be publicly exported. Solves this error:

```
"mean" is not exported from module "torch.masked" Pylance[reportPrivateImportUsage]
```

Related: https://github.com/pytorch/pytorch/pulls?q=pyright+export

Pull Request resolved: https://github.com/pytorch/pytorch/pull/131288
Approved by: https://github.com/ezyang

test/allowlist_for_publicAPI.json

@@ -476,6 +476,31 @@
     "vecdot",
     "vector_norm"
   ],
+  "torch.masked": [
+    "amax",
+    "amin",
+    "argmax",
+    "argmin",
+    "as_masked_tensor",
+    "cumprod",
+    "cumsum",
+    "is_masked_tensor",
+    "log_softmax",
+    "logaddexp",
+    "logsumexp",
+    "masked_tensor",
+    "MaskedTensor",
+    "mean",
+    "median",
+    "norm",
+    "normalize",
+    "prod",
+    "softmax",
+    "softmin",
+    "std",
+    "sum",
+    "var"
+  ],
   "torch.multiprocessing": [
     "Array",
     "AuthenticationError",

torch/masked/__init__.py

@@ -31,8 +31,27 @@ from torch.masked.maskedtensor.creation import as_masked_tensor, masked_tensor
 
 
 __all__ = [
+    "amax",
+    "amin",
+    "argmax",
+    "argmin",
     "as_masked_tensor",
+    "cumprod",
+    "cumsum",
     "is_masked_tensor",
+    "log_softmax",
+    "logaddexp",
+    "logsumexp",
     "masked_tensor",
     "MaskedTensor",
+    "mean",
+    "median",
+    "norm",
+    "normalize",
+    "prod",
+    "softmax",
+    "softmin",
+    "std",
+    "sum",
+    "var",
 ]