Add a SliceUpdate op and primitive (#850)

* Enable copy to work with int64 strides
* Fix uniform buffer indices or copy kernel arguments
* Update utils.h
* Remove manual unrolling of elem to loc loop
* GPU copy updated to handle negative strides
* Add slice update primitive

mlx/utils.cpp

@@ -329,4 +329,13 @@ std::ostream& operator<<(std::ostream& os, const std::vector<size_t>& v) {
   return os;
 }
 
+std::ostream& operator<<(std::ostream& os, const std::vector<int64_t>& v) {
+  os << "(";
+  for (int i = 0; i < v.size(); ++i) {
+    os << v[i] << ((i == v.size() - 1) ? "" : ",");
+  }
+  os << ")";
+  return os;
+}
+
 } // namespace mlx::core