Add support for Idefics 2

[EricLBuehler]

This PR adds support for our first multimodal model: Idefics 2 (https://huggingface.co/HuggingFaceM4/idefics2-8b)!

Implementation TODOs:

• VisionTransformer
  • Encoder
    • Attention
    • MLP
  • VisionEmbedding
• Connector
  • MLP
  • PerceiverLayer
• Model
  • Forward pass
    • Remove padding images
    • Generate the patch/pixel attention mask
    • Run vision submodel and connector submodel
      • Allow Mistral to run trained embedding head on any input tokens
      • Inputs merger to inject embeddings correctly
    • Pass input to Mistral model
      • Allow Mistral to take an embeddings vector instead of using the trained embedding head.
• Image processor analogous to Idefics2ImageProcessor
  • Resizing
  • Rescaling
  • Normalization
  • Padding
    • Generate pixel attention mask for padded images
      • Pass and use in input injection
  • Create pixel values tensors
• Vision Model Pipeline
  • Add a VisionModel trait similar to NormalModel
  • Add a ModelCategory: vision, text, embedding etc
  • Handle sequence scheduling with image dimensions
  • Abstract input preparation logic
    • Handle padding to same, resized shape, across batch dimension
• Add proper handling of chat templates
  • Load preprocessor/processor config JSON files
  • Support configuration of inputs processor via preprocessor
• Messages API generalization
  • Support OpenAI compatible method of specifying images
  • Update messages to optionally encode type (akin to examples here).
  • Use processor config to abstract the chat template application process
• HTTP API
  • Handle decoding from base64
  • Support loading from HTTP.
• Rust API
• Python API

Other TODOs:

• Introduce model type enum to reject mixing of text/multimodal models in speculative decoding
  • Perhaps introduce VisionModel akin to NormalModel.
• Ergonomic API support (OpenAI compatible on the HTTP side, but hopefully nicer on the Rust/Python side)
• Support device mapping
• Support ISQ

Pending issues:

• CUBLAS_STATUS_NOT_SUPPORTED for Conv2d huggingface/candle#2218

[github-actions]

Code Metrics Report

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 Language            Files        Lines         Code     Comments       Blanks
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 Happy                   1          442          369            0           73
 JSON                    9           21           21            0            0
 Python                 31         1217         1038           37          142
 TOML                   16          440          400            1           39
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 Jupyter Notebooks       1            0            0            0            0
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 (Total)                            156          117           23           16
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 Markdown               16         1149            0          846          303
 |- BASH                 5          100           97            0            3
 |- Python               6          122          110            0           12
 |- Rust                 2           80           72            3            5
 (Total)                           1451          279          849          323
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 Rust                  115        34412        31161          585         2666
 |- Markdown            57          641           13          594           34
 (Total)                          35053        31174         1179         2700
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 Total                 191        37715        33014         1469         3232
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