[bugfix] fix the complex and potentially problematic generate_kv_idx. (#5957)

### What this PR does / why we need it?
In long-sequence scenarios, the chunked-prefill component may encounter
dimension misalignment issues, which previously occurred during
precision testing on the code_generate_lite dataset. This PR removes
redundant computations and instead derives the value using existing
results and straightforward calculations.
- vLLM version: v0.13.0
- vLLM main:
2c24bc6996

Signed-off-by: QiuChunshuo <qiuchunshuo@huawei.com>

tests/e2e/multicard/4-cards/long_sequence/test_chunked_prefill.py

@@ -87,7 +87,6 @@ def test_models_chunked_prefill_mixed_length_prompts_including_1_token(
         "VLLM_ALLOW_LONG_MAX_MODEL_LEN": "1"
     })
 @pytest.mark.parametrize("model", MODELS)
-@pytest.mark.skip(reason="skip for bad adaptability with main2main")
 def test_models_chunked_prefill_with_empty_kvcache(model: str):
     TEST_ROPE_PARAMETERS = {
         "rope_theta": 1000000,