use flash attn for decode

Signed-off-by: jiqing-feng <jiqing.feng@intel.com>

Author: jiqing-feng

optimum/exporters/ipex/modeling_utils.py

@@ -652,18 +652,19 @@ def attention_interface(
                 is_causal=True,
             )
             self.use_sdpa = True
-        elif self.has_flash_attn(query) and past_len == 0:
-            # prefill, remove padding
+        elif self.has_flash_attn(query):
             attn_output = torch.empty_like(query)
             seq_len_tensor = torch.cat((input_lens.new_tensor([0]), input_lens.cumsum(-1).int()))
+            query_len_tensor = seq_len_tensor if past_len == 0 else torch.arange(seq_len_tensor.shape[0])
+            query_max_len = input_lens.max() if past_len == 0 else 1
             PagedAttention.flash_attn_varlen_func(
                 attn_output,
                 query.contiguous() if query.device.type == "xpu" else query,
                 key_cache.contiguous() if key_cache.device.type == "xpu" else key_cache,
                 value_cache.contiguous() if value_cache.device.type == "xpu" else value_cache,
+                query_len_tensor,
                 seq_len_tensor,
-                seq_len_tensor,
-                input_lens.max(),
+                query_max_len,
                 input_lens.max(),
                 1.0 / math.sqrt(self.head_dim),
                 True,