fix requirement of mlm:model role, required only by at least one asset

[fmigneault]

Description

Fixed

• Fix the validation strategy of the mlm:model role required by at least one Asset under a STAC Item.
  Although the role requirement was validated, the definition did not allow for other Assets without it to exist.

For example, the following would be rejected, because the ["metadata"] did not include the mlm:model role, while the original intention was that at least one offers it, which is respected by the other asset.

{
  "assets": {
    "card": {
      "type": "text/markdown",
      "href": "https://huggingface.co/example/model-card",
      "roles": ["metadata"],
      "title": "Model Card",
      "description": "Example model."
    },
    "model": {
      "type": "application/octet-stream; application=pytorch",
      "href": "https://example.com/sample/checkpoint.pt",
      "roles": ["checkpoint", "mlm:model"],
      "title": "Model Weights Checkpoint"
    }
  }
}

Related Issue

n/a

Type of Change

• 📚 Examples, docs, tutorials or dependencies update;
• 🔧 Bug fix (non-breaking change which fixes an issue);
• 🥂 Improvement (non-breaking change which improves an existing feature);
• 🚀 New feature (non-breaking change which adds functionality);
• 💥 Breaking change (fix or feature that would cause existing functionality to change);
• 🔐 Security fix.

Checklist

• I've read the CONTRIBUTING.md guide;
• I've updated the code style using make codestyle;
• I've written tests for all new methods and classes that I created;
• I've written the docstring in Google format for all the methods and classes that I used.