onnx: support negative index in Gather (#2440)

index_select does not support negative indexing, but
this change adds just enough workarounds in onnx to
allow evaluating silero-vad models (which make use of
negative indices).

candle-onnx/src/eval.rs

@@ -629,6 +629,18 @@ fn simple_eval_(
                 let axis = get_attr_opt::<i64>(node, "axis")?.copied().unwrap_or(0);
                 let axis = xs.normalize_axis(axis)?;
 
+                // index_select does not support negative indices, so normalize them
+                // to positive indices.
+                let indices = &{
+                    let zeros = Tensor::zeros(indices.shape(), indices.dtype(), indices.device())?;
+                    let max = Tensor::new(xs.dims()[axis] as i64, indices.device())?
+                        .to_dtype(indices.dtype())?;
+                    let mask = indices.lt(&zeros)?;
+                    mask.to_dtype(indices.dtype())?
+                        .broadcast_mul(&max)?
+                        .add(&indices)?
+                };
+
                 // In Pytorch or Numpy this can be done by indexing the xs tensor using the indices
                 // tensor directly, but candle does not support tensor indexing at the moment, so
                 // some workarounds must be done.