Fix Metal matmul failure in ModernBertHead::forward by ensuring contiguous input

[whitebox2]

Summary

On Apple Metal, ModernBertHead::forward may receive a non-contiguous view from upstream pooling (CLS/MEAN). Passing that view directly into Linear causes the Metal matmul path to misinterpret shapes and fail with:

Invalid matmul arguments [17920, 1] [1, 256] (2, 256, 256)

CPU succeeds because it tolerates non-contiguous inputs. We materialize the input with contiguous() before applying Linear. This resolves the Metal-only failure and aligns with prior guidance that GPU matmul expects contiguous tensors. ([GitHub][1])

Patch

 impl Module for ModernBertHead {
     fn forward(&self, xs: &Tensor) -> Result<Tensor> {
-        let xs = xs.apply(&self.dense)?.gelu_erf()?.apply(&self.norm)?;
+        let xs = xs.contiguous()?.apply(&self.dense)?.gelu_erf()?.apply(&self.norm)?;
         Ok(xs)
     }
 }

Issue

#3138 (comment)