Daemon plot_labels() for faster start (#9057)

* Daemon `plot_labels()` for faster start

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update train.py

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

utils/callbacks.py

@@ -3,6 +3,8 @@
 Callback utils
 """
 
+import threading
+
 
 class Callbacks:
     """"
@@ -55,17 +57,20 @@ class Callbacks:
         """
         return self._callbacks[hook] if hook else self._callbacks
 
-    def run(self, hook, *args, **kwargs):
+    def run(self, hook, *args, thread=False, **kwargs):
         """
-        Loop through the registered actions and fire all callbacks
+        Loop through the registered actions and fire all callbacks on main thread
 
         Args:
             hook: The name of the hook to check, defaults to all
             args: Arguments to receive from YOLOv5
+            thread: (boolean) Run callbacks in daemon thread
             kwargs: Keyword Arguments to receive from YOLOv5
         """
 
         assert hook in self._callbacks, f"hook '{hook}' not found in callbacks {self._callbacks}"
-
         for logger in self._callbacks[hook]:
-            logger['callback'](*args, **kwargs)
+            if thread:
+                threading.Thread(target=logger['callback'], args=args, kwargs=kwargs, daemon=True).start()
+            else:
+                logger['callback'](*args, **kwargs)