attempt to set start method to spwan to prevent cuda issues for DPO

src/axolotl/core/trainer_builder.py

@@ -13,6 +13,7 @@ from abc import abstractmethod
 from collections import defaultdict
 from dataclasses import dataclass, field
 from functools import wraps
+from multiprocessing import set_start_method
 from pathlib import Path
 from typing import Dict, List, Literal, Optional, Type, Union
 
@@ -1770,6 +1771,9 @@ class HFRLTrainerBuilder(TrainerBuilderBase):
         for callback in self.get_post_trainer_create_callbacks(dpo_trainer):
             dpo_trainer.add_callback(callback)
 
+        # prevents multiprocessing issues for datasets on multiple GPUs
+        set_start_method("spawn")
+
         return dpo_trainer