Cannot download CIFAR-10

```rust
fn main() -> Result<(), Box> {
let cifar = candle_datasets::vision::cifar::load()?;

dbg!(&cifar.train_images); // Tensor[dims 50000, 3, 32, 32; u8]
dbg!(&cifar.train_labels); // Tensor[dims 50000; u8]

dbg!(cifar
.train_images
.flatten_all()?
.to_vec1::()?
.into_iter()
.any(|x| x != 0)); // false

dbg!(cifar
.train_labels
.flatten_all()?
.to_vec1::()?
.into_iter()
.any(|x| x != 0)); // true

Ok(())
}
```

I was able to see download progress bars for `plain_text/test/0000.parquet` and `lain_text/train/0000.parquet`, but no for images.

Start by reproducing the example through candle_datasets::vision::cifar::load and inspect the CIFAR-10 download path. Compare the downloaded image data with the labels and verify that image downloads show progress and contain nonzero values.

Written by the indexing model from the issue text.