onnx: fix pad, unsqueeze (#2317)

* onnx: fix pad, unsqueeze

both implementations have off-by-one errors:
- Pad 'reflect' cycle for eg `dim==3` is `[0,1,2,1]` which has length of
  4 (or `dim*2 - 2`) not 5 (current code `dim*2 - 1`)
- Unsqueeze(-1) for tensor with `dim==3` should be 3 (ie `dim+index+1`)
  not 2 (ie currently `dim+index`)

in addition, Pad is incorrectly calculating the starting padding.
If we want to pad out 2 elements to the start, and we have this cycle
of indices of length 6, then we should skip 4 elements, but currently
we skip 2. A more visual representation of what's going on is below:

```
pad_start: 2
data:      [a,b,c,d]
indices:   [0, 1, 2, 3, 2, 1, 0, 1, 2, 3, 2, 1, 0, ..] // zigzag between 0..4
actual:    skip [ c  d| c  b  a  b]
expected:  ~  skip  ~ [ c  b| a  b  c  d]
```

The values between `[` and `|` are padding and the values between
`|` and `]` in the example should match the original data being padded.

* Fix clippy lints.

---------

Co-authored-by: Laurent <laurent.mazare@gmail.com>

candle-examples/examples/yolo-v8/main.rs

@@ -390,7 +390,7 @@ pub fn run<T: Task>(args: Args) -> anyhow::Result<()> {
     for image_name in args.images.iter() {
         println!("processing {image_name}");
         let mut image_name = std::path::PathBuf::from(image_name);
-        let original_image = image::io::Reader::open(&image_name)?
+        let original_image = image::ImageReader::open(&image_name)?
             .decode()
             .map_err(candle::Error::wrap)?;
         let (width, height) = {