Use Llama RMSNorm custom op for Gemma (vllm-project#2974)

Author: WoosukKwon

vllm/model_executor/models/gemma.py

@@ -22,6 +22,7 @@
 
 from vllm.model_executor.input_metadata import InputMetadata
 from vllm.model_executor.layers.attention import PagedAttention
+from vllm.model_executor.layers.layernorm import RMSNorm
 from vllm.model_executor.layers.linear import (ColumnParallelLinear,
                                                LinearMethodBase,
                                                QKVParallelLinear,
@@ -40,21 +41,6 @@
 KVCache = Tuple[torch.Tensor, torch.Tensor]
 
 
-class GemmaRMSNorm(nn.Module):
-
-    def __init__(self, dim: int, eps: float = 1e-6):
-        super().__init__()
-        self.eps = eps
-        self.weight = nn.Parameter(torch.zeros(dim))
-
-    def _norm(self, x):
-        return x * torch.rsqrt(x.pow(2).mean(-1, keepdim=True) + self.eps)
-
-    def forward(self, x):
-        output = self._norm(x.float()).type_as(x)
-        return output * (1 + self.weight)
-
-
 class GemmaMLP(nn.Module):
 
     def __init__(
@@ -185,36 +171,38 @@ def __init__(
             intermediate_size=config.intermediate_size,
             linear_method=linear_method,
         )
-        self.input_layernorm = GemmaRMSNorm(config.hidden_size,
-                                            eps=config.rms_norm_eps)
-        self.post_attention_layernorm = GemmaRMSNorm(config.hidden_size,
-                                                     eps=config.rms_norm_eps)
+        self.input_layernorm = RMSNorm(config.hidden_size,
+                                       eps=config.rms_norm_eps)
+        self.post_attention_layernorm = RMSNorm(config.hidden_size,
+                                                eps=config.rms_norm_eps)
 
     def forward(
         self,
         positions: torch.Tensor,
         hidden_states: torch.Tensor,
         kv_cache: KVCache,
         input_metadata: InputMetadata,
+        residual: Optional[torch.Tensor],
     ) -> Tuple[torch.Tensor, torch.Tensor]:
         # Self Attention
-        residual = hidden_states
-        hidden_states = self.input_layernorm(hidden_states)
+        if residual is None:
+            residual = hidden_states
+            hidden_states = self.input_layernorm(hidden_states)
+        else:
+            hidden_states, residual = self.input_layernorm(
+                hidden_states, residual)
         hidden_states = self.self_attn(
             positions=positions,
             hidden_states=hidden_states,
             kv_cache=kv_cache,
             input_metadata=input_metadata,
         )
-        hidden_states = residual + hidden_states
 
         # Fully Connected
-        residual = hidden_states
-        hidden_states = self.post_attention_layernorm(hidden_states)
+        hidden_states, residual = self.post_attention_layernorm(
+            hidden_states, residual)
         hidden_states = self.mlp(hidden_states)
-        hidden_states = residual + hidden_states
-
-        return hidden_states
+        return hidden_states, residual
 
 
 class GemmaModel(nn.Module):
@@ -235,7 +223,7 @@ def __init__(
             GemmaDecoderLayer(config, linear_method)
             for _ in range(config.num_hidden_layers)
         ])
-        self.norm = GemmaRMSNorm(config.hidden_size, eps=config.rms_norm_eps)
+        self.norm = RMSNorm(config.hidden_size, eps=config.rms_norm_eps)
 
     def forward(
         self,
@@ -246,17 +234,19 @@ def forward(
     ) -> torch.Tensor:
         hidden_states = self.embed_tokens(input_ids)
         # Normalize the embedding by sqrt(hidden_size)
-        hidden_states = hidden_states * (self.config.hidden_size**0.5)
+        hidden_states *= self.config.hidden_size**0.5
 
+        residual = None
         for i in range(len(self.layers)):
             layer = self.layers[i]
-            hidden_states = layer(
+            hidden_states, residual = layer(
                 positions,
                 hidden_states,
                 kv_caches[i],
                 input_metadata,
+                residual,
             )
-        hidden_states = self.norm(hidden_states)
+        hidden_states, _ = self.norm(hidden_states, residual)
         return hidden_states
 
 
@@ -321,6 +311,10 @@ def load_weights(self,
                 # Skip loading extra layer for lora models.
                 if "lm_head" in name:
                     continue
+                # GemmaRMSNorm is different from Llama's in that it multiplies
+                # (1 + weight) to the output, instead of just weight.
+                if "norm.weight" in name:
+                    loaded_weight += 1.0
                 param = params_dict[name]
                 weight_loader = getattr(param, "weight_loader",
                                         default_weight_loader)
@@ -329,5 +323,5 @@ def load_weights(self,
         unloaded_params = params_dict.keys() - loaded_params
         if unloaded_params:
             raise RuntimeError(
-                f"Some weights are not initialized from checkpoints: {unloaded_params}"
-            )
+                "Some weights are not initialized from checkpoints: "
+                f"{unloaded_params}")