message: DiffArray::nbytes() implementation
diff: @@ -1622,6 +1622,13 @@ public:
  ENOKI_INLINE const Value &value_() const { return value; }
  ENOKI_INLINE Value &value_() { return value; }
  + size_t nbytes() const {
  + if constexpr (is_array_v<Value>)
  + return sizeof(DiffArray) - sizeof(Value) + value.nbytes();
  + else
  + return sizeof(DiffArray);
  + }
  +
  const Value &gradient_() const { return gradient_static_(index); }
  ENOKI_NOINLINE static const Value &gradient_static_(Index index) {
  