[chat] fix enable single gpu training bug

applications/Chat/examples/train_prompts.py

@@ -8,7 +8,7 @@ from coati.models.bloom import BLOOMRM, BLOOMActor, BLOOMCritic
 from coati.models.gpt import GPTRM, GPTActor, GPTCritic
 from coati.models.llama import LlamaActor, LlamaCritic, LlamaRM
 from coati.models.opt import OPTRM, OPTActor, OPTCritic
-from coati.models.roberta import RoBERTaRM, RoBERTaActor, RoBERTaCritic
+from coati.models.roberta import RoBERTaActor, RoBERTaCritic, RoBERTaRM
 from coati.trainer import PPOTrainer
 from coati.trainer.strategies import ColossalAIStrategy, DDPStrategy, NaiveStrategy
 from coati.utils import prepare_llama_tokenizer_and_embedding
@@ -143,6 +143,8 @@ def main(args):
     prompt_dataset = PromptDataset(tokenizer=tokenizer, data_path=args.prompt_path, max_datasets_size=16384)
     if dist.is_initialized() and dist.get_world_size() > 1:
         prompt_sampler = DistributedSampler(prompt_dataset, shuffle=True, seed=42, drop_last=True)
+    else:
+        prompt_sampler = None
     prompt_dataloader = DataLoader(prompt_dataset,
                                    shuffle=(prompt_sampler is None),
                                    sampler=prompt_sampler,
@@ -151,6 +153,8 @@ def main(args):
     pretrain_dataset = SupervisedDataset(tokenizer=tokenizer, data_path=args.pretrain_dataset, max_datasets_size=16384)
     if dist.is_initialized() and dist.get_world_size() > 1:
         pretrain_sampler = DistributedSampler(pretrain_dataset, shuffle=True, seed=42, drop_last=True)
+    else:
+        pretrain_sampler = None
     pretrain_dataloader = DataLoader(pretrain_dataset,
                                      shuffle=(pretrain_sampler is None),
                                      sampler=pretrain_sampler,