Fix missing imports and code formatting of `llava/train/llava_trainer.py`

[naufalso]

Summary

Resolve import errors encountered when running scripts/train/pretrain_clip.sh by adding the required imports and applying minor formatting updates in llava/train/llava_trainer.py. No functional changes intended beyond unblocking the training run.

Changes

• Add missing imports in llava/train/llava_trainer.py
• Apply black code formatting

Testing

• Ran bash scripts/train/pretrain_clip.sh; training starts and runs without import errors.