[Executorch][llama] Refactoring sdpa

- Change variable name from is_sdpa_with_kv_cache to is_seq_at_dim_1 to be more meaningful
- remove seq_len as it can be derived from the size of query

Differential Revision: [D62623243](https://our.internmc.facebook.com/intern/diff/D62623243/)

ghstack-source-id: 244549439
Pull Request resolved: #5619

Author: kimishpatel

extension/llm/custom_ops/op_sdpa.cpp

@@ -224,7 +224,7 @@ void cpu_flash_attention(
     bool is_causal,
     const optional<Tensor>& attn_mask,
     const optional<double>& scale,
-    bool is_with_kv_cache = false,
+    bool is_seq_at_dim_1 = false,
     const int64_t start_pos = 0) {
   (void)dropout_p;
   // Query (Batch x Num_heads  x Q_seq_len  x Dim_per_head)
@@ -265,7 +265,7 @@ void cpu_flash_attention(
   int64_t kvSize = value.size(2);
   int64_t num_heads_kv = key.size(1);
 
-  if (is_with_kv_cache) {
+  if (is_seq_at_dim_1) {
     num_head = query.size(2);
     num_heads_kv = key.size(2);
     qSize = query.size(1);
@@ -311,7 +311,7 @@ void cpu_flash_attention(
   int64_t qStrideH = strides[1];
   int64_t qStrideM = strides[2];
 
-  if (is_with_kv_cache) {
+  if (is_seq_at_dim_1) {
     qStrideH = strides[2];
     qStrideM = strides[1];
   }
@@ -321,7 +321,7 @@ void cpu_flash_attention(
   int64_t kStrideH = strides[1];
   int64_t kStrideN = strides[2];
 
-  if (is_with_kv_cache) {
+  if (is_seq_at_dim_1) {
     kStrideH = strides[2];
     kStrideN = strides[1];
   }
@@ -331,7 +331,7 @@ void cpu_flash_attention(
   int64_t vStrideH = strides[1];
   int64_t vStrideN = strides[2];
 
-  if (is_with_kv_cache) {
+  if (is_seq_at_dim_1) {
     vStrideH = strides[2];
     vStrideN = strides[1];
   }
@@ -341,7 +341,7 @@ void cpu_flash_attention(
   int64_t oStrideH = strides[1];
   int64_t oStrideM = strides[2];
 
-  if (is_with_kv_cache) {
+  if (is_seq_at_dim_1) {
     oStrideH = strides[2];
     oStrideM = strides[1];
   }
@@ -776,7 +776,6 @@ Tensor& custom_sdpa_out(
     const Tensor& k,
     const Tensor& v,
     const int64_t start_pos,
-    const int64_t seq_len,
     const optional<Tensor>& attn_mask,
     const double dropout_p,
     const bool is_causal,
@@ -792,6 +791,7 @@ Tensor& custom_sdpa_out(
 
   ET_CHECK_MSG(q.dim() == 4, "query must be a 4D tensor");
 
+  const int64_t seq_len = q.size(1);
   auto q_seq_len = q.size(1);
 
   // Refactor the following into create_view util perhaps using
@@ -870,7 +870,7 @@ Tensor& custom_sdpa_out(
           is_causal,
           attn_mask,
           scale,
-          true,
+          true, /* is_seq_at_dim_1 */
           start_pos);
     } else if (q_seq_len >= 192) {
       cpu_flash_attention<CTYPE, 64, 512>(
@@ -882,7 +882,7 @@ Tensor& custom_sdpa_out(
           is_causal,
           attn_mask,
           scale,
-          true,
+          true, /* is_seq_at_dim_1 */
           start_pos);
     } else {
       cpu_flash_attention<CTYPE, 32, 512>(
@@ -894,7 +894,7 @@ Tensor& custom_sdpa_out(
           is_causal,
           attn_mask,
           scale,
-          true,
+          true, /* is_seq_at_dim_1 */
           start_pos);
     }
   });
@@ -1017,7 +1017,6 @@ Tensor& sdpa_with_kv_cache_out(
       key_cache,
       value_cache,
       start_pos,
-      seq_len,
       attn_mask,
       dropout_p,
       is_causal,