Add sum

Author: saulshanabrook

python/egglog/exp/array_api.py

@@ -1272,6 +1272,9 @@ def size(self) -> Int: ...
     def __len__(self) -> int:
         return self.size.eval()
 
+    @method(egg_fn="sum")
+    def sum(self, axis: OptionalIntOrTuple = None) -> NDArray: ...
+
     @method(preserve=True)
     def __iter__(self) -> Iterator[NDArray]:
         for i in range(len(self)):