Remove uses of ATOMIC_VAR_INIT

ATOMIC_VAR_INIT has a trivial definition
`#define ATOMIC_VAR_INIT(value) (value)`,
is deprecated in C17/C++20, and will be removed in newer standards in
newer GCC/Clang (e.g. https://reviews.llvm.org/D144196).

PiperOrigin-RevId: 525534393

mediapipe/framework/scheduler.h

@@ -310,7 +310,7 @@ class Scheduler {
   absl::Mutex state_mutex_;
 
   // Current state of the scheduler.
-  std::atomic<State> state_ = ATOMIC_VAR_INIT(STATE_NOT_STARTED);
+  std::atomic<State> state_ = STATE_NOT_STARTED;
 
   // True if all graph input streams are closed.
   bool graph_input_streams_closed_ ABSL_GUARDED_BY(state_mutex_) = false;