[Model] Add Gemma 2 (#5908)

vllm/model_executor/layers/layernorm.py

@@ -95,3 +95,49 @@ class RMSNorm(CustomOp):
         s = f"hidden_size={self.weight.data.size(0)}"
         s += f", eps={self.variance_epsilon}"
         return s
+
+
+class GemmaRMSNorm(CustomOp):
+    """RMS normalization for Gemma.
+
+    Two differences from the above RMSNorm:
+        1. x * (1 + w) instead of x * w.
+        2. (x * w).to(orig_dtype) instead of x.to(orig_dtype) * w.
+    """
+
+    def __init__(
+        self,
+        hidden_size: int,
+        eps: float = 1e-6,
+    ) -> None:
+        super().__init__()
+        self.weight = nn.Parameter(torch.zeros(hidden_size))
+        self.variance_epsilon = eps
+
+    def forward_native(
+        self,
+        x: torch.Tensor,
+        residual: Optional[torch.Tensor] = None,
+    ) -> Union[torch.Tensor, Tuple[torch.Tensor, torch.Tensor]]:
+        """PyTorch-native implementation equivalent to forward()."""
+        orig_dtype = x.dtype
+        if residual is not None:
+            x = x + residual
+            residual = x
+
+        x = x.float()
+        variance = x.pow(2).mean(dim=-1, keepdim=True)
+        x = x * torch.rsqrt(variance + self.variance_epsilon)
+        # Llama does x.to(float16) * w whilst Gemma is (x * w).to(float16)
+        # See https://github.com/huggingface/transformers/pull/29402
+        x = x * (1.0 + self.weight.float())
+        x = x.to(orig_dtype)
+        return x if residual is None else (x, residual)
+
+    def forward_cuda(
+        self,
+        x: torch.Tensor,
+        residual: Optional[torch.Tensor] = None,
+    ) -> Union[torch.Tensor, Tuple[torch.Tensor, torch.Tensor]]:
+        # TODO(woosuk): Implement an optimized kernel for GemmaRMSNorm.
+        return self.forward_native(x, residual)