NNUE namespace cleanup

This patch moves the nnue namespace in the appropiate header that correspondes with the definition.
It also makes navigation a bit easier.

closes https://github.com/official-stockfish/Stockfish/pull/4445

No functional change

src/nnue/evaluate_nnue.h

@@ -55,6 +55,14 @@ namespace Stockfish::Eval::NNUE {
   template <typename T>
   using LargePagePtr = std::unique_ptr<T, LargePageDeleter<T>>;
 
+  std::string trace(Position& pos);
+  Value evaluate(const Position& pos, bool adjusted = false, int* complexity = nullptr);
+  void hint_common_parent_position(const Position& pos);
+
+  bool load_eval(std::string name, std::istream& stream);
+  bool save_eval(std::ostream& stream);
+  bool save_eval(const std::optional<std::string>& filename);
+
 }  // namespace Stockfish::Eval::NNUE
 
 #endif // #ifndef NNUE_EVALUATE_NNUE_H_INCLUDED