Add Mistral3 vision-language model support (For Flux2 Migration)

[SpenserCai]

Summary

This PR adds support for the Mistral3 (Mistral-Small-3.x) vision-language model to candle-transformers. Mistral3 combines the Pixtral vision encoder with the Mistral language model, enabling multimodal image-text understanding.

Note: This PR is a preparatory step for the upcoming Flux2 model migration, as Flux2 shares similar multimodal architecture patterns with Mistral3.

Changes

New files in candle-transformers/src/models/mistral3/:

• mod.rs - Module exports and documentation
• config.rs - Mistral3Config with vision, text, and projector settings
• model.rs - Mistral3Model and Mistral3ForConditionalGeneration
• patch_merger.rs - PatchMerger for reducing image tokens
• projector.rs - MultiModalProjector (RMSNorm + PatchMerger + MLP)

Modified files:

• candle-transformers/src/models/mod.rs - Added mistral3 module export
• candle-transformers/src/models/pixtral/vision_model.rs - Added forward_with_hidden_states() and VisionModelOutput struct
• candle-transformers/src/models/mistral.rs - Added forward_embeds_hidden() for multimodal integration

Architecture

Mistral3ForConditionalGeneration
├── Mistral3Model
│   ├── vision_tower (Pixtral Vision Model, 24 layers)
│   ├── multi_modal_projector
│   │   ├── norm (RMSNorm)
│   │   ├── patch_merger (spatial_merge_size=2, reduces tokens by 4x)
│   │   ├── linear_1
│   │   ├── act (GELU)
│   │   └── linear_2
│   └── language_model (Mistral, 40 layers)
└── lm_head

Key Implementation Details

1. PatchMerger: Uses reshape + permute to implement PyTorch's unfold operation (kernel_size == stride, no overlap), merging 2x2 patches into one.

2. Image Token Replacement: Implements replace_image_tokens() as Candle equivalent of PyTorch's masked_scatter.

3. Vision Tower Integration: Uses forward_with_hidden_states() to get batch-dimension-preserved output matching PyTorch Transformers behavior.

Supported Models

• mistralai/Mistral-Small-3.1-24B-Instruct-2503
• mistralai/Mistral-Small-3.2-24B-Instruct-2506

Differences from Pixtral LLaVA

Feature	Pixtral LLaVA	Mistral3
PatchMerger	❌	✅ (spatial_merge_size=2)
Projector RMSNorm	❌	✅
Projector bias	✅	❌
Image token reduction	1x	4x

Usage

use candle_transformers::models::mistral3::{Mistral3Config, Mistral3ForConditionalGeneration};

let config: Mistral3Config = serde_json::from_str(&config_str)?;
let model = Mistral3ForConditionalGeneration::new(&config, vb)?;
let logits = model.forward(&input_ids, Some(&pixel_values), Some(&image_sizes), 0)?;

Verification

The implementation has been verified against PyTorch Transformers reference:

• Vision Tower: avg_diff = 2.29e-4
• MultiModal Projector: avg_diff = 3.61e-8
• Full Forward Pass: Predicted token matches (token ID: 1784 "The")

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Checklist

• New model implementation follows existing patterns in candle-transformers
• Configuration uses serde for JSON deserialization
• Reuses existing components (Pixtral vision, Mistral language model)
• Documentation comments included
• Verified against PyTorch reference implementation

[SpenserCai]

mistral3 examples added!

[SpenserCai]

Fixed clippy and fmt.