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Fix RMSNorm fp32 input mutation #205

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Fix RMSNorm fp32 input mutation #205

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2 changes: 1 addition & 1 deletion nanovllm/layers/layernorm.py
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Original file line number Diff line number Diff line change
Expand Up @@ -21,7 +21,7 @@ def rms_forward(
orig_dtype = x.dtype
x = x.float()
var = x.pow(2).mean(dim=-1, keepdim=True)
x.mul_(torch.rsqrt(var + self.eps))
x = x * torch.rsqrt(var + self.eps)
x = x.to(orig_dtype).mul_(self.weight)
return x

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23 changes: 23 additions & 0 deletions test/test_layernorm.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,23 @@
import importlib.util
from pathlib import Path

import torch


spec = importlib.util.spec_from_file_location(
"layernorm",
Path(__file__).parents[1] / "nanovllm" / "layers" / "layernorm.py",
)
layernorm = importlib.util.module_from_spec(spec)
spec.loader.exec_module(layernorm)
RMSNorm = layernorm.RMSNorm


def test_rms_norm_does_not_modify_fp32_input():
norm = RMSNorm(4)
x = torch.randn(2, 4, dtype=torch.float32)
expected = x.clone()

norm(x)

torch.testing.assert_close(x, expected)