[ROCm] Upgrade transformers to v4.38.0 (vllm-project#2967)

Author: WoosukKwon

requirements-rocm.txt

@@ -6,7 +6,7 @@ ray >= 2.9
 sentencepiece  # Required for LLaMA tokenizer.
 numpy
 tokenizers>=0.15.0
-transformers >= 4.37.0  # Required for Mixtral.
+transformers >= 4.38.0  # Required for Gemma.
 fastapi
 uvicorn[standard]
 pydantic >= 2.0  # Required for OpenAI server.