Fix incorrect reflect padding in tensors with 3+ dims

When reading the start padding values from the `padding` input, read the last 2
values from the first half of the vector, instead of the first two.

src/ops/pad.rs

@@ -78,9 +78,12 @@ pub fn pad<T: Copy>(
 
             let pad_dims = input.ndim() - batch_dims;
             let (pad_top, pad_left) = if pad_dims == 1 {
-                (0, padding[[0]] as usize)
+                (0, padding[[batch_dims]] as usize)
             } else {
-                (padding[[0]] as usize, padding[[1]] as usize)
+                (
+                    padding[[batch_dims]] as usize,
+                    padding[[batch_dims + 1]] as usize,
+                )
             };
 
             let mut input = input.view();
@@ -338,12 +341,24 @@ mod tests {
                 pads: NdTensor::from([]),
                 expected: Ok(Tensor::from(2.)),
             },
+            // Pad start columns of a 3D tensor.
+            Case {
+                input: [[[1., 2., 3.]]].into(),
+                pads: [0, 0, 2, 0, 0, 0].into(),
+                expected: Ok(Tensor::from([[[3., 2., 1., 2., 3.]]])),
+            },
             // Pad end columns of a 3D tensor.
             Case {
                 input: [[[1., 2., 3.]]].into(),
                 pads: [0, 0, 0, 0, 0, 2].into(),
                 expected: Ok(Tensor::from([[[1., 2., 3., 2., 1.]]])),
             },
+            // Pad start rows of a 3D tensor.
+            Case {
+                input: [[[1.], [2.], [3.]]].into(),
+                pads: [0, 2, 0, 0, 0, 0].into(),
+                expected: Ok(Tensor::from([[[3.], [2.], [1.], [2.], [3.]]])),
+            },
             // Pad channel dimension of a 3D tensor.
             Case {
                 input: [[[1., 2., 3.]]].into(),