Revisit NNUE initialization

this revisits the initialization of NNUE, basically only changing
the state on the UCI options 'Use NNUE' and 'EvalFile' calling init_NNUE(),
which sets the Eval::useNNUE variable, and loads the network if needed
(i.e. useNNUE is true and the same network is not yet loaded)

init_NNUE is silent (i.e. no info strings), so that it can be called at startup
without confusing certain GUIs.

An error message on wrong setting when asking for (i.e. the net failed to load),
is delayed to the point where everything must be consistent (start of search or eval).
The engine will stop if the settings are wrong at that point.

Also works if the default value of Use NNUE would become true.

src/ucioption.cpp

@@ -42,23 +42,8 @@ void on_hash_size(const Option& o) { TT.resize(size_t(o)); }
 void on_logger(const Option& o) { start_logger(o); }
 void on_threads(const Option& o) { Threads.set(size_t(o)); }
 void on_tb_path(const Option& o) { Tablebases::init(o); }
-
-void on_use_nnue(const Option& o) {
-
-  if (o)
-      std::cout << "info string NNUE eval used" << std::endl;
-  else
-      std::cout << "info string classic eval used" << std::endl;
-
-  Eval::useNNUE = o;
-  init_nnue(Options["EvalFile"]);
-}
-
-void on_eval_file(const Option& o) {
-
-  load_eval_finished = false;
-  init_nnue(o);
-}
+void on_use_NNUE(const Option& ) { Eval::init_NNUE(); }
+void on_eval_file(const Option& ) { Eval::init_NNUE(); }
 
 /// Our case insensitive less() function as required by UCI protocol
 bool CaseInsensitiveLess::operator() (const string& s1, const string& s2) const {
@@ -95,7 +80,7 @@ void init(OptionsMap& o) {
   o["SyzygyProbeDepth"]      << Option(1, 1, 100);
   o["Syzygy50MoveRule"]      << Option(true);
   o["SyzygyProbeLimit"]      << Option(7, 0, 7);
-  o["Use NNUE"]              << Option(false, on_use_nnue);
+  o["Use NNUE"]              << Option(false, on_use_NNUE);
   o["EvalFile"]              << Option("nn-c157e0a5755b.nnue", on_eval_file);
 }
 
@@ -205,5 +190,4 @@ Option& Option::operator=(const string& v) {
   return *this;
 }
 
-bool load_eval_finished = false;
 } // namespace UCI