Set workers=0 for MPS Train and Val modes (#4697)

ultralytics/utils/callbacks/neptune.py

@@ -1,10 +1,6 @@
 # Ultralytics YOLO 🚀, AGPL-3.0 license
 
-import matplotlib.image as mpimg
-import matplotlib.pyplot as plt
-
 from ultralytics.utils import LOGGER, SETTINGS, TESTS_RUNNING
-from ultralytics.utils.torch_utils import model_info_for_loggers
 
 try:
     assert not TESTS_RUNNING  # do not log pytest
@@ -13,11 +9,12 @@ try:
     from neptune.types import File
 
     assert hasattr(neptune, '__version__')
+
+    run = None  # NeptuneAI experiment logger instance
+
 except (ImportError, AssertionError):
     neptune = None
 
-run = None  # NeptuneAI experiment logger instance
-
 
 def _log_scalars(scalars, step=0):
     """Log scalars to the NeptuneAI experiment logger."""
@@ -42,6 +39,9 @@ def _log_plot(title, plot_path):
         title (str) Title of the plot
         plot_path (PosixPath or str) Path to the saved image file
         """
+    import matplotlib.image as mpimg
+    import matplotlib.pyplot as plt
+
     img = mpimg.imread(plot_path)
     fig = plt.figure()
     ax = fig.add_axes([0, 0, 1, 1], frameon=False, aspect='auto', xticks=[], yticks=[])  # no ticks
@@ -70,6 +70,7 @@ def on_train_epoch_end(trainer):
 def on_fit_epoch_end(trainer):
     """Callback function called at end of each fit (train+val) epoch."""
     if run and trainer.epoch == 0:
+        from ultralytics.utils.torch_utils import model_info_for_loggers
         run['Configuration/Model'] = model_info_for_loggers(trainer)
     _log_scalars(trainer.metrics, trainer.epoch + 1)