Fix LSE output error in FA2 kvsplit (#87)

Signed-off-by: griii <guo_rui@mail.ustc.edu.cn>

Author: griii

csrc/flash_attn/flash_api.cpp

@@ -765,7 +765,18 @@ mha_varlen_fwd(at::Tensor &q,  // total_q x num_heads x head_size, total_q := \s
         // q_padded = q_padded.reshape(size_before).transpose(1, 2).reshape(size_after);
         int64_t lse_size_before[] = {num_heads, batch_size, max_seqlen_q};
         int64_t lse_size_after[] = {num_heads * max_seqlen_q, batch_size};
-        softmax_lse = softmax_lse.reshape(lse_size_before).transpose(1, 2).reshape(lse_size_after);
+        
+        
+        if (params.num_splits > 1){
+            // When KV-split is enabled (num_splits > 1), LSE is first computed partially through lse_accum tensors. Then, an additional kernel, combine_attn_seqk_parallel, reduces these partials into the final LSE. 
+            // This kernel produces LSE in a [seqlen_q, h, b] layout which can be directly used as it is already in the canonical form.
+            softmax_lse = softmax_lse.reshape(lse_size_after); 
+        }else{
+            // The standard forward kernel produces LSE in a [b, h, seqlen_q] layout.
+            // It must be transposed to the canonical [seqlen_q, h, b] layout.
+            softmax_lse = softmax_lse.reshape(lse_size_before).transpose(1, 2).reshape(lse_size_after);
+        }
+        
     }
 
     return {out, softmax_lse};