ultralytics 8.0.101 mosaic9() and loss bug fixes (#2608)

Co-authored-by: Laughing <61612323+Laughing-q@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Yonghye Kwon <developer.0hye@gmail.com>

ultralytics/yolo/engine/trainer.py

@@ -181,6 +181,8 @@ class BaseTrainer:
             # Command
             cmd, file = generate_ddp_command(world_size, self)
             try:
+                LOGGER.info('Pre-caching dataset to avoid NCCL timeout before running DDP command')
+                deepcopy(self)._setup_train(world_size=0)
                 LOGGER.info(f'Running DDP command {cmd}')
                 subprocess.run(cmd, check=True)
             except Exception as e: