Char datatype assumption

[green-br]

Just building vllm on aarch64 and see the following warning during the build process:

  /vllm/.deps/flashmla-src/csrc/flash_api.cpp: In function ‘std::vector<at::Tensor> get_mla_metadata(at::Tensor&, int, int)’:         
  /vllm/.deps/flashmla-src/csrc/flash_api.cpp:46:38: warning: narrowing conversion of ‘(char)(& seqlens_k)->at::Tensor::<anonymous>.at::TensorBase::get_device()’ from ‘char’ to ‘c10::DeviceIndex’ {aka ‘signed char’} [-Wnarrowing]                                                                                         46 |     at::cuda::CUDAGuard device_guard{(char)seqlens_k.get_device()};                                                                                                              |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                                                                          
/vllm/.deps/flashmla-src/csrc/flash_api.cpp: In function ‘std::vector<at::Tensor> mha_fwd_kvcache_mla(at::Tensor&, const at::Tensor&, int, const at::Tensor&, const at::Tensor&, float, bool, const at::Tensor&, const at::Tensor&)’:                                                                                  
  /vllm/.deps/flashmla-src/csrc/flash_api.cpp:133:38: warning: narrowing conversion of ‘(char)(& q)->at::Tensor::<anonymous>.at::TensorBase::get_device()’ from ‘char’ to ‘c10::DeviceIndex’ {aka ‘signed char’} [-Wnarrowing]                                                                                           
    133 |     at::cuda::CUDAGuard device_guard{(char)q.get_device()};                                                                                                              
        |                                      ^~~~~~~~~~~~~~~~~~~~          

Since on aarch64 char is by default unsigned compared to x86 where it is signed I think we need to be a bit more precise in which char is being used to remove the warning. Would using (signed char) be enough?