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/src/imagenet.rs

@@ -3,7 +3,7 @@ use candle::{Device, Result, Tensor};
 /// Loads an image from disk using the image crate at the requested resolution.
 //  This returns a tensor with shape (3, res, res). imagenet normalization is applied.
 pub fn load_image<P: AsRef<std::path::Path>>(p: P, res: u32) -> Result<Tensor> {
-    let img = image::io::Reader::open(p)?
+    let img = image::ImageReader::open(p)?
         .decode()
         .map_err(candle::Error::wrap)?
         .resize_to_fill(res, res, image::imageops::FilterType::Triangle);

candle-examples/src/lib.rs

@@ -34,7 +34,7 @@ pub fn load_image<P: AsRef<std::path::Path>>(
     p: P,
     resize_longest: Option<usize>,
 ) -> Result<(Tensor, usize, usize)> {
-    let img = image::io::Reader::open(p)?
+    let img = image::ImageReader::open(p)?
         .decode()
         .map_err(candle::Error::wrap)?;
     let (initial_h, initial_w) = (img.height() as usize, img.width() as usize);
@@ -65,7 +65,7 @@ pub fn load_image_and_resize<P: AsRef<std::path::Path>>(
     width: usize,
     height: usize,
 ) -> Result<Tensor> {
-    let img = image::io::Reader::open(p)?
+    let img = image::ImageReader::open(p)?
         .decode()
         .map_err(candle::Error::wrap)?
         .resize_to_fill(