Support MPT with GQA (#1938)

Co-authored-by: Woosuk Kwon <[email protected]>

Author: megha95

vllm/model_executor/layers/attention.py

@@ -138,7 +138,8 @@ def forward(
                     input_metadata.attn_bias = attn_bias
                 else:
                     input_metadata.attn_bias = _make_alibi_bias(
-                        self.alibi_slopes, batch_size, seq_len, query.dtype)
+                        self.alibi_slopes, self.num_kv_heads, batch_size,
+                        seq_len, query.dtype)
 
             # TODO(woosuk): Too many view operations. Let's try to reduce them
             # in the future for code readability.
@@ -180,31 +181,34 @@ def forward(
 
 def _make_alibi_bias(
     alibi_slopes: torch.Tensor,
+    num_kv_heads: int,
     batch_size: int,
     seq_len: int,
     dtype: torch.dtype,
 ) -> LowerTriangularMaskWithTensorBias:
-    bias = torch.arange(seq_len, dtype=dtype)
+    bias = torch.arange(seq_len, dtype=dtype, device="cuda")
     # NOTE(zhuohan): HF uses
     #     `bias = bias[None, :].repeat(prompt_len, 1)`
     # here. We find that both biases give the same results, but
     # the bias below more accurately follows the original ALiBi
     # paper.
     bias = bias[None, :] - bias[:, None]
-    bias = bias.to(alibi_slopes.device)
 
     # When using custom attention bias, xformers requires the bias to
     # be sliced from a tensor whose length is a multiple of 8.
     padded_len = (seq_len + 7) // 8 * 8
+    num_heads = alibi_slopes.shape[0]
     bias = torch.empty(
         batch_size,
-        alibi_slopes.shape[0],
+        num_heads,
         seq_len,
         padded_len,
         device=alibi_slopes.device,
         dtype=dtype,
     )[:, :, :, :seq_len].copy_(bias)
     bias.mul_(alibi_slopes[:, None, None])
+    if num_heads != num_kv_heads:
+        bias = bias.unflatten(1, (num_kv_heads, num_heads // num_kv_heads))
     attn_bias = LowerTriangularMaskWithTensorBias(bias)
     return attn_bias
 

vllm/model_executor/models/mpt.py

@@ -50,9 +50,14 @@ def __init__(
         super().__init__()
         self.d_model = config.d_model
         self.total_num_heads = config.n_heads
+        self.head_dim = self.d_model // self.total_num_heads
         self.clip_qkv = config.attn_config["clip_qkv"]
         self.qk_ln = config.attn_config["qk_ln"]
         self.alibi_bias_max = config.attn_config["alibi_bias_max"]
+        if "kv_n_heads" in config.attn_config:
+            self.total_num_kv_heads = config.attn_config['kv_n_heads']
+        else:
+            self.total_num_kv_heads = self.total_num_heads
         assert not config.attn_config["prefix_lm"]
         assert config.attn_config["alibi"]
 
@@ -61,6 +66,7 @@ def __init__(
             self.d_model,
             self.d_model // self.total_num_heads,
             self.total_num_heads,
+            self.total_num_kv_heads,
             bias=not config.no_bias,
             linear_method=linear_method,
         )
@@ -78,6 +84,17 @@ def __init__(
         assert self.total_num_heads % tp_world_size == 0
         self.num_heads = self.total_num_heads // tp_world_size
 
+        if self.total_num_kv_heads >= tp_world_size:
+            # Number of KV heads is greater than TP size, so we partition
+            # the KV heads across multiple tensor parallel GPUs.
+            assert self.total_num_kv_heads % tp_world_size == 0
+        else:
+            # Number of KV heads is less than TP size, so we replicate
+            # the KV heads across multiple tensor parallel GPUs.
+            assert tp_world_size % self.total_num_kv_heads == 0
+        self.num_kv_heads = max(1, self.total_num_kv_heads // tp_world_size)
+        self.q_size = self.num_heads * self.head_dim
+        self.kv_size = self.num_kv_heads * self.head_dim
         # Create the alibi slopes and slice them.
         tp_rank = get_tensor_model_parallel_rank()
         head_start = tp_rank * self.num_heads
@@ -91,7 +108,8 @@ def __init__(
         self.attn = PagedAttention(self.num_heads,
                                    self.head_dim,
                                    scaling,
-                                   alibi_slopes=alibi_slopes)
+                                   alibi_slopes=alibi_slopes,
+                                   num_kv_heads=self.num_kv_heads)
 
     def forward(
         self,
@@ -105,7 +123,7 @@ def forward(
         qkv, _ = self.Wqkv(hidden_states)
         if self.clip_qkv is not None:
             qkv.clamp_(min=-self.clip_qkv, max=self.clip_qkv)
-        q, k, v = qkv.chunk(chunks=3, dim=-1)
+        q, k, v = qkv.split([self.q_size, self.kv_size, self.kv_size], dim=-1)
         if self.qk_ln:
             q = self.q_ln(q)
             k = self.k_ln(k)