ONNX support for scalar unsqueeze

[antimora]

There is case when we need to unsqueeze scalar. This came about in #1560. See ONNX graph screenshot down below.

Pull Request Template

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

1. Support for 0-Dimensional Tensors in Burn #1689
2. Convert ONNX graph inputs of 0-dim tensors to scalars #1688
3. ONNX conversion: Only tensor input is valid Argument #1560

Changes

1. Added Ignored new-type to ignore types in the module.
2. Updated Model module to store device
3. Updated unsqueeze to deal with scalar input type.
4. Updated tests

Testing

1. New unit tests
2. Tested user onnx file
3. run-checks.sh

[laggui]

I'm somewhat ambivalent about these changes. Obviously, to support unsqueeze for scalars right now we have to do something like this.

But at the same time I feel like this is gonna come back to haunt us for other reasons 😅

This is just very isolated, and with the state of other ops that have different behavior for scalars (e.g., some just capture the value in a 1D tensor with a single element).

As an isolated change to support unsqueeze for scalars, this is fine. I'll leave my review as a comment for now, interested in hearing feedback from others (@nathanielsimard and yourself included)

[nathanielsimard]

Same comment as @laggui, I think this is fine in the short term, but supporting scalar tensors from onnx without having to sync the device is very valuable, we'll have to think about how we can do it easily.