[CI] Revert back prepare_prompts and check_answers (#25087)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

tests/models/test_transformers.py

@@ -8,8 +8,7 @@ import pytest
 from vllm.platforms import current_platform
 
 from ..conftest import HfRunner, VllmRunner
-from ..core.block.e2e.test_correctness_sliding_window import prep_prompts
-from ..utils import multi_gpu_test
+from ..utils import multi_gpu_test, prep_prompts
 from .utils import check_logprobs_close