Make result of AtScopeExit non-discardable. (#6248)

* Make result of AtScopeExit non-discardable.
* Make CallAtExit constructor nodiscard.

---------
Signed-off-by: Michal Zientkiewicz <michalz@nvidia.com>

Author: mzient

include/dali/core/call_at_exit.h

@@ -1,4 +1,4 @@
-// Copyright (c) 2022, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// Copyright (c) 2022-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@ namespace detail {
  */
 template <typename Callable>
 struct CallAtExit {
+  [[nodiscard("The callback would be called immediately.")]]
   explicit CallAtExit(Callable &&c) : callable(std::move(c)) {}
   ~CallAtExit() noexcept(false) {
     callable();
@@ -35,6 +36,7 @@ struct CallAtExit {
 }  // namespace detail
 
 template <typename Callable>
+[[nodiscard("The callback would be called immediately.")]]
 detail::CallAtExit<Callable> AtScopeExit(Callable &&c) {
   return detail::CallAtExit<Callable>(std::forward<Callable>(c));
 }