fix diagonal bias

Author: timt51

csrc/flash_attn/src/mask.h

@@ -153,7 +153,7 @@ struct Mask {
                         for (int mi = 0; mi < size<0>(tensor); ++mi) {
                             // No causal, no local
                             if constexpr (Has_alibi) {
-                                tensor(mi, make_coord(j, nj)) += alibi_slope; // * col_idx;
+                                tensor(mi, make_coord(j, nj)) += (col_idx == 0 ? 0 : alibi_slope); // alibi_slope * col_idx;
                             }
                             if constexpr (!Is_even_MN) {
                                 if (col_idx >= max_seqlen_k) { tensor(mi, make_coord(j, nj)) = -INFINITY; }
@@ -178,9 +178,10 @@ struct Mask {
                                 const int col_idx = col_idx_base + j;
                                 if constexpr (Has_alibi) {
                                     if constexpr (Is_causal) {
-                                        tensor(make_coord(i, mi), make_coord(j, nj)) += alibi_slope; // * col_idx;
+                                        tensor(make_coord(i, mi), make_coord(j, nj)) += (col_idx == row_idx ? 0 : alibi_slope); // alibi_slope * col_idx;
+
                                     } else {
-                                        tensor(make_coord(i, mi), make_coord(j, nj)) -= alibi_slope; // * abs(row_idx + max_seqlen_k - max_seqlen_q - col_idx);
+                                        tensor(make_coord(i, mi), make_coord(j, nj)) += (col_idx == row_idx ? 0 : alibi_slope); // -= alibi_slope * abs(row_idx + max_seqlen_k - max_seqlen_q - col_idx);
 
                                     }
                                 }