[rocm] fix rocm setup.py build

[draftbk]

Purpose

I am encountering an issue that is very likely caused by this PR (#23791) when building on AMD MI300X.

# Run
python setup.py clean && python setup.py build
# Error
FAILED: [code=1] CMakeFiles/_C.dir/csrc/cache_kernels.hip.o

/data/users/lifans/gitrepos/vllm/build/temp.linux-x86_64-cpython-312/csrc/cache_kernels.hip:918:14: error: unused variable 'is_last_split' [-Werror,-Wunused-variable]
  918 |   const bool is_last_split = (split_end == tot_slots);
      |              ^~~~~~~~~~~~~
2 warnings and 1 error generated when compiling for gfx942.

Test Plan & Test Result

# python setup.py clean && python setup.py build
The error is gone. 

# Run 
from vllm import LLM
if __name__ == "__main__":
    llm = LLM("/home/lifans/hf_models/meta-llama/Llama-3.1-8B-Instruct")
    result = llm.generate("Hello my name is")
    print(result[0].outputs[0].text)

# Output
Adding requests: 100%|█████████████████| 1/1 [00:00<00:00, 55.36it/s]
Processed prompts: 100%|█| 1/1 [00:01<00:00,  1.19s/it, est. speed in
 Helen and I am a 35 year old mother of three. I am a

[gemini-code-assist]

Code Review

This pull request correctly resolves a build failure on ROCm platforms by removing the unused variable is_last_split from the cp_gather_cache kernel. The change is minimal and directly addresses the -Werror,-Wunused-variable compilation error. The logic of the kernel is unaffected as the variable was not used. The fix is approved.

[draftbk]

cc: @youkaichao @gshtras

[yuguo68]

already fixed in #23847

[draftbk]

already fixed in #23847

oh coool, thanks, close this PR