LLM: add flash attention support for llama (#9518)

* add initial flash attention for llama

* accelerate fp32 first token by changing to fp16 in advance

* support fp32

python/llm/src/bigdl/llm/transformers/models/llama.py

@@ -106,6 +106,16 @@ def llama_attention_forward_4_31(
 ) -> Tuple[torch.Tensor, Optional[torch.Tensor], Optional[Tuple[torch.Tensor]]]:
     bsz, q_len, _ = hidden_states.size()
     device = hidden_states.device
+    # for flash attention
+    original_dtype = hidden_states.dtype
+    if not self.training and not hidden_states.requires_grad:
+        fsdp_flag = check_flash_attention_available(hidden_states)
+    else:
+        fsdp_flag = False
+    if fsdp_flag and q_len > 1:
+        attention_dtype = torch.float16  # use fp16 for flash attention
+    else:
+        attention_dtype = original_dtype
 
     if self.config.pretraining_tp > 1:
         key_value_slicing = (self.num_key_value_heads * self.head_dim) // self.config.pretraining_tp
@@ -194,31 +204,23 @@ def llama_attention_forward_4_31(
 
     # repeat k/v heads if n_kv_heads < n_heads
     key_states = repeat_kv(key_states, self.num_key_value_groups).to(device,
-                                                                     dtype=hidden_states.dtype)
+                                                                     dtype=attention_dtype)
     value_states = repeat_kv(value_states, self.num_key_value_groups).to(device,
-                                                                         dtype=hidden_states.dtype)
+                                                                         dtype=attention_dtype)
 
-    attn_weights = torch.matmul(query_states,
-                                key_states.transpose(2, 3)) / math.sqrt(self.head_dim)
-
-    attn_weights_size = (bsz, self.num_heads, q_len, kv_seq_len)
-    if attn_weights.size() != attn_weights_size:
-        invalidInputError(False,
-                          f"Attention weights should be of size {attn_weights_size}, "
-                          f"but is {attn_weights.size()}")
-
-    if attention_mask is not None:
-        attn_mask_size = (bsz, 1, q_len, kv_seq_len)
-        if attention_mask.size() != attn_mask_size:
-            invalidInputError(False,
-                              f"Attention mask should be of size {attn_mask_size}, "
-                              f"but is {attention_mask.size()}")
-        attn_weights = attn_weights + attention_mask
-
-    # upcast attention to fp32
-    attn_weights = nn.functional.softmax(attn_weights, dim=-1,
-                                         dtype=torch.float32).to(query_states.dtype)
-    attn_output = torch.matmul(attn_weights, value_states)
+    if fsdp_flag and q_len > 1:
+        # now only use flash attention for first token
+        attn_output = F.scaled_dot_product_attention(query_states.to(dtype=attention_dtype),
+                                                     key_states,
+                                                     value_states,
+                                                     is_causal=True)
+        attn_weights = None
+    else:
+        # otherwise, use native attention
+        attn_output, attn_weights = native_sdp(query_states, key_states, value_states,
+                                               attention_mask,
+                                               bsz, q_len, kv_seq_len,
+                                               self.head_dim, self.num_heads)
 
     attn_output_size = (bsz, self.num_heads, q_len, self.head_dim)
     if attn_output.size() != attn_output_size:
@@ -241,4 +243,46 @@ def llama_attention_forward_4_31(
     if not output_attentions:
         attn_weights = None
 
-    return attn_output, attn_weights, past_key_value
+    return attn_output.to(original_dtype), attn_weights, past_key_value
+
+
+def check_flash_attention_available(query):
+    # check whether ipex flash attention can be used
+    if query.device.type != "xpu":
+        # ipex flash attention only support for xpu
+        return False
+    ipex_version = get_ipex_version()
+    if ipex_version <= "2.0.110+xpu":
+        # ipex flash attention is supported from ipex 2.1
+        return False
+    if not torch.xpu.has_xetla():
+        # ipex flash attention is only supported for xetla
+        # may update this later
+        return False
+    return True
+
+
+def native_sdp(query, key, value, attention_mask,
+               bsz, q_len, kv_seq_len, head_dim, num_heads):
+    attn_weights = torch.matmul(query,
+                                key.transpose(2, 3)) / math.sqrt(head_dim)
+
+    attn_weights_size = (bsz, num_heads, q_len, kv_seq_len)
+    if attn_weights.size() != attn_weights_size:
+        invalidInputError(False,
+                          f"Attention weights should be of size {attn_weights_size}, "
+                          f"but is {attn_weights.size()}")
+
+    if attention_mask is not None:
+        attn_mask_size = (bsz, 1, q_len, kv_seq_len)
+        if attention_mask.size() != attn_mask_size:
+            invalidInputError(False,
+                              f"Attention mask should be of size {attn_mask_size}, "
+                              f"but is {attention_mask.size()}")
+        attn_weights = attn_weights + attention_mask
+
+    # upcast attention to fp32
+    attn_weights = nn.functional.softmax(attn_weights, dim=-1,
+                                         dtype=torch.float32).to(value.dtype)
+    attn_output = torch.matmul(attn_weights, value)
+    return attn_output, attn_weights