feat: MiniCPM-V-2.6 serving

[PanZezhong1725]

Summary

• Adds MiniCPM-V multimodal request support around image inputs.
• Introduces a dedicated MiniCPMVProcessor that:
  -- loads HF AutoProcessor / tokenizer,
  -- applies MiniCPM-V chat templates,
  -- processes image_url messages,
  -- builds batched model inputs including pixel_values, tgt_sizes, image_bound, and image request ids.
• Changes multimodal input plumbing from single tensors to per-request tensor lists:
  -- pixel_values
  -- image_bound
  -- tgt_sizes
  -- image_req_ids
• Updates C++ InferEngine, RankWorker, pybind bindings, and InfinilmModel::Input to carry those per-request multimodal tensors.
• Adds forward-context multimodal metadata so C++ model code can know which scheduled requests contain image data.
• Updates MiniCPMVModel so image embeddings are replaced inside the already-batched token embeddings using request offsets, instead of assuming a single request.
• Fixes MiniCPM-V batching behavior for continuous batching / paged attention.
• Extends prefix-cache hashing to include multimodal identifiers, preventing different images with similar token text from incorrectly sharing cached blocks.
• Adds logic to avoid reusing cache blocks that partially cover incomplete multimodal spans.
• Optimizes MiniCPM-V vision path:
  -- resampler now precomputes a max-size 2D sin/cos embedding table,
  -- SigLIP attention switches to fused QKV projection,
  -- SigLIP can use the configured attention backend, including flash attention.
• Makes fused linear helpers easier to use without explicit quantization by defaulting to NoneQuantization.
• Adds OpenAI-style image request tooling:
  -- test/service/request.py
  -- image-aware scripts/test_perf.py
  -- README example for single-request image service testing.
• Adjusts examples/test_infer.py to insert image content into the conversation structure correctly.

Motivation

Closes #343

Type of Change

• feat — new feature / new model
• fix — bug fix
• perf — performance improvement (no behavioral change)
• refactor — code restructuring without behavior change
• test — adding or fixing tests only
• docs — documentation only
• build / ci — build system or CI configuration
• chore — tooling, formatting, or other non-code changes
• Breaking change

Test Results of Involved Models on Supported Platforms (Please attach screenshots)

• MiniCPM-V-2.6 serving:

• 9g8b test_infer

• qwen3 bench

Benchmark / Performance Impact

Notes for Reviewers

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Testing

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• Passed single request test (examples/test_infer.py), or specify the reason for skipping.
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