[ET-VK][ez] Fix IndexError in Vulkan partitioner DtypeSetList/TensorRepSetList

The `__getitem__` methods of `DtypeSetList` and `TensorRepSetList` in
`utils.py` could raise an `IndexError` when the index is greater than or
equal to the length of the list. This can happen when partitioning ops
whose number of inputs or outputs exceeds the number of entries in the
dtype/tensor-rep specification list. Fix by returning an empty set in
this case, matching the intent of the existing broadcasting logic.

Differential Revision: [D95970163](https://our.internmc.facebook.com/intern/diff/D95970163/)

[ghstack-poisoned]

Author: ssjia

backends/vulkan/utils.py

@@ -91,6 +91,8 @@ def __getitem__(self, idx: int) -> DtypeSet:
         # Broadcasting: single set applies to all positions
         if idx > 0 and len(self.vals) == 1:
             return self.vals[0]
+        if idx >= len(self.vals):
+            return set()
         return self.vals[idx]
 
     def is_empty(self) -> bool:
@@ -1227,8 +1229,9 @@ def __len__(self):
     def __getitem__(self, idx: int) -> TensorRepSet:
         if idx > 0 and len(self) == 1:
             return self.vals[0]
-        else:
-            return self.vals[idx]
+        if idx >= len(self.vals):
+            return set()
+        return self.vals[idx]
 
     def __setitem__(self, idx: int, val: TensorRepSet) -> None:
         if idx > 0 and len(self.vals) == 1: