I don’t know what nn.LayerNorm is doing:

```
x=to.arange(4).float()
l=nn.LayerNorm(4, elementwise_affine=True)
# I expected the below value = 1
(l(x)*x.std(-1)/(x-x.mean(-1, keepdim=True)))[0]
```

But this code generates:

`1.1547`

I think it is:

`np.sqrt(4)/np.sqrt(3) = 1.1547005383792517`

Why the LayerNorm multiply this prefactor??