optimize our iterator to avoid an unecessary constructor call (#741)

Author: luitjens

include/matx/core/iterator.h

@@ -98,7 +98,15 @@ struct RandomOperatorIterator {
 
   [[nodiscard]] __MATX_INLINE__ __MATX_HOST__ __MATX_DEVICE__ value_type operator[](difference_type offset) const
   {
-    return *self_type{t_, offset_ + offset};
+    if constexpr (OperatorType::Rank() == 0) {
+      return static_cast<value_type>(t_.operator()());
+    }
+    else {
+      auto arrs = detail::GetIdxFromAbs(t_, offset_+offset);
+      return cuda::std::apply([&](auto &&...args) {
+          return static_cast<value_type>(t_.operator()(args...));
+        }, arrs);     
+    }
   }  
 
   __MATX_INLINE__ __MATX_HOST__ __MATX_DEVICE__  self_type operator++(int)