[perf] enable multiproc dataloader in sft trainer (#122)

- Without multiproc

Train 1/2: 1%|▍ | 20/3934 [01:38<5:14:50, 4.83s/it

Avg GPU utilization: 55%

- With multiproc

Train 1/2: 1%|▍ | 20/3934 [01:00<2:57:09, 2.72s/it]

Avg GPU utilization: 95%

verl/trainer/fsdp_sft_trainer.py

@@ -120,6 +120,8 @@ class FSDPSFTTrainer(object):
         self.train_dataloader = DataLoader(dataset=self.train_dataset,
                                            batch_size=config.data.train_batch_size,
                                            sampler=self.train_sampler,
+                                           num_workers=8,
+                                           pin_memory=True,
                                            drop_last=True)
 
         self.val_sampler = DistributedSampler(self.val_dataset,
@@ -130,6 +132,8 @@ class FSDPSFTTrainer(object):
         self.val_dataloader = DataLoader(dataset=self.val_dataset,
                                          batch_size=config.data.micro_batch_size,
                                          sampler=self.val_sampler,
+                                         num_workers=8,
+                                         pin_memory=True,
                                          drop_last=True)
 
     def _build_model_optimizer(self):