Add Mistral3 multimodal support with Pixtral vision encoder

This adds support for Mistral3 multimodal models (vision + text):

- `Bumblebee.Vision.Pixtral`: Pixtral vision encoder with RoPE support
- `Bumblebee.Text.Mistral3`: Mistral3 text decoder with interleaved attention
- `Bumblebee.Multimodal.Mistral3`: Vision-language model combining Pixtral
  and Mistral3 with multimodal projector for image-conditioned generation

Supported architectures:
- PixtralVisionModel
- Mistral3Model, Mistral3ForCausalLM, Mistral3ForSequenceClassification
- Mistral3ForConditionalGeneration (multimodal)

Author: dai-yamashita

lib/bumblebee.ex

@@ -170,6 +170,12 @@ defmodule Bumblebee do
     "MistralModel" => {Bumblebee.Text.Mistral, :base},
     "MistralForCausalLM" => {Bumblebee.Text.Mistral, :for_causal_language_modeling},
     "MistralForSequenceClassification" => {Bumblebee.Text.Mistral, :for_sequence_classification},
+    "Mistral3Model" => {Bumblebee.Text.Mistral3, :base},
+    "Mistral3ForCausalLM" => {Bumblebee.Text.Mistral3, :for_causal_language_modeling},
+    "Mistral3ForSequenceClassification" =>
+      {Bumblebee.Text.Mistral3, :for_sequence_classification},
+    "Mistral3ForConditionalGeneration" =>
+      {Bumblebee.Multimodal.Mistral3, :for_conditional_generation},
     "PhiModel" => {Bumblebee.Text.Phi, :base},
     "PhiForCausalLM" => {Bumblebee.Text.Phi, :for_causal_language_modeling},
     "PhiForSequenceClassification" => {Bumblebee.Text.Phi, :for_sequence_classification},
@@ -198,6 +204,7 @@ defmodule Bumblebee do
     "T5Model" => {Bumblebee.Text.T5, :base},
     "T5ForConditionalGeneration" => {Bumblebee.Text.T5, :for_conditional_generation},
     "T5EncoderModel" => {Bumblebee.Text.T5, :encoder},
+    "PixtralVisionModel" => {Bumblebee.Vision.Pixtral, :base},
     "ViTForImageClassification" => {Bumblebee.Vision.Vit, :for_image_classification},
     "ViTForMaskedImageModeling" => {Bumblebee.Vision.Vit, :for_masked_image_modeling},
     "ViTModel" => {Bumblebee.Vision.Vit, :base},
@@ -255,6 +262,7 @@ defmodule Bumblebee do
     "layoutlm" => :layout_lm,
     "llama" => :llama,
     "mistral" => :llama,
+    "mistral3" => :llama,
     "mbart" => :mbart,
     "phi" => :code_gen,
     "phi3" => :llama,