[AMP] support setting amp_level in multi-thread (#39198)

Author: zhiqiu

paddle/fluid/imperative/tracer.cc

@@ -32,6 +32,8 @@ namespace imperative {
 
 thread_local bool Tracer::has_grad_ = true;
 
+thread_local AmpLevel Tracer::amp_level_ = AmpLevel::O0;
+
 static std::shared_ptr<Tracer> g_current_tracer(nullptr);
 
 const std::shared_ptr<Tracer>& GetCurrentTracer() { return g_current_tracer; }

paddle/fluid/imperative/tracer.h

@@ -126,7 +126,7 @@ class Tracer {
   platform::Place expected_place_;
   GarbageCollectorMap gcs_;
   static thread_local bool has_grad_;
-  AmpLevel amp_level_{AmpLevel::O0};
+  static thread_local AmpLevel amp_level_;
 };
 
 // To access static variable current_tracer