[NDMatrix] Resolved Dangling Reference Warning

GCC13 introduced a warning for potentially dangling references. The
NDMatrix class was creating temporary objects to store the offseted
pointer and then returning refrences to these pointers. Although this is
correct, since the pointer is owned by the base NDMatrix object, it was
confusing the compiler which is not something you want to do in general.

Changed how the NDMatrix proxy class works by storing the offset into
the pointer directly and passing a reference to the base pointer
instead. This makes it clear to the compiler that this pointer belongs
to another class and we are returning a reference to a portion of that
memory, which is allowed.

libs/libvtrutil/src/vtr_ndmatrix.h

@@ -34,9 +34,10 @@ class NdMatrixProxy {
      *    @param dim_stride: The stride of this dimension (i.e. how many element in memory between indicies of this dimension)
      *    @param  start: Pointer to the start of the sub-matrix this proxy represents
      */
-    NdMatrixProxy(const size_t* dim_sizes, const size_t* dim_strides, T* start)
+    NdMatrixProxy(const size_t* dim_sizes, const size_t* dim_strides, size_t offset, const std::unique_ptr<T[]>& start)
         : dim_sizes_(dim_sizes)
         , dim_strides_(dim_strides)
+        , offset_(offset)
         , start_(start) {}
 
     NdMatrixProxy& operator=(const NdMatrixProxy& other) = delete;
@@ -50,7 +51,8 @@ class NdMatrixProxy {
         return NdMatrixProxy<T, N - 1>(
             dim_sizes_ + 1,                    // Pass the dimension information
             dim_strides_ + 1,                  // Pass the stride for the next dimension
-            start_ + dim_strides_[0] * index); // Advance to index in this dimension
+            offset_ + dim_strides_[0] * index, // Advance to index in this dimension
+            start_);                           // Pass the base pointer.
     }
 
     ///@brief [] operator
@@ -62,7 +64,8 @@ class NdMatrixProxy {
   private:
     const size_t* dim_sizes_;
     const size_t* dim_strides_;
-    T* start_;
+    size_t offset_;
+    const std::unique_ptr<T[]>& start_;
 };
 
 ///@brief Base case: 1-dimensional array
@@ -76,9 +79,10 @@ class NdMatrixProxy<T, 1> {
      *    @param dim_stride: The stride of this dimension (i.e. how many element in memory between indicies of this dimension)
      *    @param  start: Pointer to the start of the sub-matrix this proxy represents
      */
-    NdMatrixProxy(const size_t* dim_sizes, const size_t* dim_stride, T* start)
+    NdMatrixProxy(const size_t* dim_sizes, const size_t* dim_stride, size_t offset, const std::unique_ptr<T[]>& start)
         : dim_sizes_(dim_sizes)
         , dim_strides_(dim_stride)
+        , offset_(offset)
         , start_(start) {}
 
     NdMatrixProxy& operator=(const NdMatrixProxy& other) = delete;
@@ -89,7 +93,7 @@ class NdMatrixProxy<T, 1> {
         VTR_ASSERT_SAFE_MSG(index < dim_sizes_[0], "Index out of range (above dimension maximum)");
 
         //Base case
-        return start_[index];
+        return start_[offset_ + index];
     }
 
     ///@brief [] operator
@@ -108,7 +112,7 @@ class NdMatrixProxy<T, 1> {
      * not to clobber elements in other dimensions
      */
     const T* data() const {
-        return start_;
+        return start_.get() + offset_;
     }
 
     ///@brief same as above but allow update the value
@@ -120,7 +124,8 @@ class NdMatrixProxy<T, 1> {
   private:
     const size_t* dim_sizes_;
     const size_t* dim_strides_;
-    T* start_;
+    size_t offset_;
+    const std::unique_ptr<T[]>& start_;
 };
 
 /**
@@ -359,7 +364,8 @@ class NdMatrix : public NdMatrixBase<T, N> {
         return NdMatrixProxy<T, N - 1>(
             this->dim_sizes_.data() + 1,                        //Pass the dimension information
             this->dim_strides_.data() + 1,                      //Pass the stride for the next dimension
-            this->data_.get() + this->dim_strides_[0] * index); //Advance to index in this dimension
+            this->dim_strides_[0] * index,                      //Advance to index in this dimension
+            this->data_);                                       //Pass the base pointer
     }
 
     /**