Enable `RMSNorm` substitution for Transformers backend (#26353)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/model_executor/models/transformers.py

@@ -43,7 +43,7 @@ from vllm.config.utils import getattr_iter
 from vllm.distributed import get_pp_group, get_tp_group
 from vllm.distributed.utils import get_pp_indices
 from vllm.logger import init_logger
-from vllm.model_executor.layers.layernorm import RMSNorm
+from vllm.model_executor.layers.layernorm import GemmaRMSNorm, RMSNorm
 from vllm.model_executor.layers.linear import (
     ColumnParallelLinear,
     ReplicatedLinear,
@@ -194,15 +194,29 @@ def replace_rms_norm_class(rms_norm: nn.Module, hidden_size: int) -> RMSNorm:
     - `var_hidden_size` is only ever used for Intern vision encoder in vLLM
     and Transformers doesn't appear to have the same concept.
     """
-    kwargs = {
-        "hidden_size": hidden_size,
-        "eps": getattr_iter(rms_norm, ("eps", "variance_epsilon"), 1e-6),
-        "has_weight": getattr(rms_norm, "with_scale", True),
-    }
-    if (weight := getattr(rms_norm, "weight", None)) is not None:
-        # If weight is a Parameter, get its data tensor
-        weight = getattr(weight, "data", weight)
-        kwargs["dtype"] = weight.dtype
+    eps = getattr_iter(rms_norm, ("eps", "variance_epsilon"), 1e-6)
+    kwargs = {"hidden_size": hidden_size, "eps": eps}
+    # Update hidden size if weight is available
+    weight_meta = getattr(rms_norm, "weight", None)
+    if weight_meta is not None:
+        kwargs["hidden_size"] = weight_meta.size(0)
+    # Check if weight is all zeros, which indicates GemmaRMSNorm
+    # We must create a new instance because rms_norm is on meta
+    try:
+        with torch.device("cpu"):
+            weight_test = getattr(rms_norm.__class__(1), "weight", None)
+    except Exception:
+        logger.warning(
+            "Failed to determine if RMSNorm weight is centered on zero or one. "
+            "Defaulting to one."
+        )
+        weight_test = None
+    if weight_test is not None and torch.all(weight_test == 0):
+        return GemmaRMSNorm(**kwargs)
+    # Otherwise assume it's a regular RMSNorm
+    kwargs["has_weight"] = getattr(rms_norm, "with_scale", True)
+    if weight_meta is not None:
+        kwargs["dtype"] = weight_meta.dtype
     else:
         # No weight, fall back to weightless RMSNorm
         kwargs["has_weight"] = False
@@ -645,11 +659,10 @@ class TransformersBase(nn.Module, SupportsQuant, SupportsLoRA, SupportsPP):
                     new_module = replace_linear_class(
                         child_module, style, self.quant_config, prefix=qual_name
                     )
-                # TODO(hmellor): Enable RMSNorm replacement once we have a way
-                # to choose RMSNorm vs GemmaRMSNorm
-                # elif child_module.__class__.__name__.endswith("RMSNorm"):
-                #     new_module = replace_rms_norm_class(
-                #         child_module, self.config.hidden_size)
+                elif child_module.__class__.__name__.endswith("RMSNorm"):
+                    new_module = replace_rms_norm_class(
+                        child_module, self.text_config.hidden_size
+                    )
                 else:
                     _recursive_replace(child_module, prefix=qual_name)