[Kernel] support merge_attn_states CUDA kernel, 3x speedup (#16173)

Signed-off-by: DefTruth <qiustudent_r@163.com>

csrc/torch_bindings.cpp

@@ -64,6 +64,21 @@ TORCH_LIBRARY_EXPAND(TORCH_EXTENSION_NAME, ops) {
       "    int blocksparse_head_sliding_step) -> ()");
   ops.impl("paged_attention_v2", torch::kCUDA, &paged_attention_v2);
 
+#ifndef USE_ROCM
+  // Merge attn states
+  // Implements section 2.2 of https://www.arxiv.org/pdf/2501.01005
+  // can be used to combine partial attention results (in the split-KV case)
+  ops.def(
+      "merge_attn_states("
+      "    Tensor! output,"
+      "    Tensor!? output_lse,"
+      "    Tensor prefix_output,"
+      "    Tensor prefix_lse,"
+      "    Tensor suffix_output,"
+      "    Tensor suffix_lse) -> ()");
+  ops.impl("merge_attn_states", torch::kCUDA, &merge_attn_states);
+#endif
+
   // Activation ops
   // Activation function used in SwiGLU.
   ops.def("silu_and_mul(Tensor! out, Tensor input) -> ()");