[misc] feat: Modify transformers dependency version in requirements (verl-project#5141)

Update transformers version constraint to be less than 5.0.0.

### What does this PR do?

- Force transformers < 5.0.0 before it is supported in verl

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Author: vermouth1992

requirements.txt

@@ -15,7 +15,7 @@ pre-commit
 ray[default]
 tensordict>=0.8.0,<=0.10.0,!=0.9.0
 torchdata
-transformers
+transformers<5.0.0
 # vllm==0.8.4
 wandb
 packaging>=20.0