Added a global (to gemma) zones list to enable most call sites to PROFILER_ZONE3 to avoid the sychronization required for the static const initialization of the zone handle.

Improved flash_attention to enable profiling using the new zones.

PiperOrigin-RevId: 817072887

Author: Highway

hwy/profiler.h

@@ -113,6 +113,7 @@ class ZoneHandle {
 
   bool operator==(const ZoneHandle other) const { return bits_ == other.bits_; }
   bool operator!=(const ZoneHandle other) const { return bits_ != other.bits_; }
+  bool IsEmpty() const { return bits_ == 0; }
 
   size_t ZoneIdx() const {
     HWY_DASSERT(bits_ != 0);