Add bidirectional attention and projection layer support for Qwen3-based models

[williambarberjr]

What does this PR do?

This PR adds support for voyageai/voyage-4-nano, a Qwen3-based embedding model that uses bidirectional attention and a projection layer.

Changes

1. Bidirectional Attention Support

• Added use_bidirectional_attention config field (default: false)
• When true, disables causal masking in the attention mechanism
• voyage-4-nano and similar embedding models use bidirectional attention to see the full context

2. Projection Layer Support

• Added num_labels config field for output projection dimension
• When set, loads linear.weight from safetensors root level and applies projection after final normalization
• voyage-4-nano projects from hidden_size=1024 to output_dim=2048

Model Configuration

Models using these features should have in their config.json:

{
  "use_bidirectional_attention": true,
  "num_labels": 2048
}

Testing

Tested with voyageai/voyage-4-nano:

• ✅ Output dimension: 2048 (correct)
• ✅ Cosine similarity vs HuggingFace transformers: 0.999965
• ✅ Inference time: ~9ms on L4 GPU (vs 35ms with transformers)

Files Changed

• backends/candle/src/models/flash_qwen3.rs - CUDA/flash attention implementation
• backends/candle/src/models/qwen3.rs - CPU/Metal implementation + config struct
• backends/candle/Cargo.toml - Added cudarc dev-dependency for CUDA tests
• backends/candle/tests/test_voyage_nano.rs - CPU test with snapshots
• backends/candle/tests/test_flash_voyage_nano.rs - CUDA test with snapshots
• README.md - Added voyage-4-nano to supported models table
• docs/source/en/supported_models.md - Added voyage-4-nano to docs

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines.
• Did you write any new necessary tests? If applicable, did you include or update the insta snapshots?

Who can review?

@Narsil @alvarobartt - This adds two new config fields to support voyage-4-nano embedding model. The changes are backwards compatible (both fields default to disabled behavior).