[Core] Factor out input preprocessing to a separate class (#7329)

tests/engine/test_skip_tokenizer_init.py

@@ -11,9 +11,10 @@ def test_skip_tokenizer_initialization(model: str):
     # token ids.
     llm = LLM(model=model, skip_tokenizer_init=True)
     sampling_params = SamplingParams(prompt_logprobs=True, detokenize=True)
-    with pytest.raises(ValueError) as err:
+
+    with pytest.raises(ValueError, match="cannot pass text prompts when"):
         llm.generate("abc", sampling_params)
-    assert "prompts must be None if" in str(err.value)
+
     outputs = llm.generate({"prompt_token_ids": [1, 2, 3]},
                            sampling_params=sampling_params)
     assert len(outputs) > 0