Remove unused variable in gqa_attn_splitk_attn_kernel (pytorch#4014)

Summary:

X-link: facebookresearch/FBGEMM#1102

ATT

Reviewed By: helunwencser

Differential Revision: D73554136

Author: PatriceVignola

fbgemm_gpu/experimental/gen_ai/src/attention/gqa_attn_splitk.cu

@@ -1066,7 +1066,6 @@ __global__ void gqa_attn_splitk_attn_kernel(
   // Each block handles a single batch and head
   auto b = blockIdx.x;
   auto h = blockIdx.y;
-  int32_t split_k = XQ_out.size(0);
 
   // Note: this is decoding case where we attent to current and all previous
   // tokens.