Add modular detector

docs/source/en/internal/model_debugging_utils.md

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 📄 JSON saved to /home/pablo/git/transformers/scan_test_inputs_embeds.json
 
 ```
+
+## Modular model detector
+
+### Code similarity analyzer - for model adders
+
+This utility analyzes code similarities between model implementations to identify opportunities for modularization. It compares a new or existing modeling file against all models in the library using embedding-based and token-based similarity metrics.
+
+### Rationale
+
+When adding a new model to transformers, many components (attention layers, MLPs, outputs, etc.) may already exist in similar form in other models. Instead of implementing everything from scratch, model adders can identify which existing classes are similar and potentially reusable through modularization.
+
+The tool computes two similarity scores:
+- **Embedding score**: Uses semantic code embeddings (via `Qwen/Qwen3-Embedding-4B`) to detect functionally similar code even with different naming
+- **Jaccard score**: Measures token set overlap to identify structurally similar code patterns
+
+A score of 1.00 means the code is identical.
+
+### Usage
+
+From the root of the `transformers` repository:
+
+```bash
+python utils/modular_model_detector.py --modeling-file path/to/modeling_file.py
+```
+
+The tool will automatically download the pre-built index from the Hub (requires RAM/VRAM for the embedding model).
+
+**Example output:**
+
+```text
+Loading checkpoint shards: 100%|████████████████████| 2/2 [00:00<00:00, 33.62it/s]
+encoding 21 query definitions with Qwen/Qwen3-Embedding-4B (device=cuda, batch=16, max_length=4096)
+
+stuff.py::Beit3ImageTextMatchingOutput:
+embedding:
+    blip_2::Blip2ImageTextMatchingModelOutput (0.9994)
+    chinese_clip::ChineseCLIPOutput (0.9818)
+    owlvit::OwlViTOutput (0.9818)
+jaccard:
+    owlv2::Owlv2Output (0.9667)
+    metaclip_2::MetaClip2Output (0.9667)
+    altclip::AltCLIPOutput (0.9667)
+intersection:
+    blip::BlipOutput
+    owlvit::OwlViTOutput
+
+stuff.py::Beit3MLP:
+embedding:
+    efficientloftr::EfficientLoFTRMLP (0.9718)
+    seggpt::SegGptMlp (0.9650)
+jaccard:
+    chinese_clip::ChineseCLIPTextSelfOutput (0.5294)
+    bert::BertSelfOutput (0.5294)
+intersection:
+```
+
+The `intersection` field shows classes that appear in both top-5 results, indicating high confidence for modularization candidates.
+
+### Building a custom index
+
+To rebuild the index from your local codebase (useful after adding new models or using a different embedding model):
+
+```bash
+python utils/modular_model_detector.py --build
+```
+
+To push the rebuilt index to a Hub dataset:
+
+```bash
+python utils/modular_model_detector.py --build --push-new-index --hub-dataset your-org/your-dataset
+```
+
+### Options
+
+- `--modeling-file`: Path to the modeling file to analyze
+- `--build`: Build the code similarity index from all modeling files in `src/transformers/models/`
+- `--push-new-index`: After building, push the index to a Hub dataset (requires `--build`)
+- `--hub-dataset`: Hub dataset repository ID to pull/push the index (default: `hf-internal-testing/transformers_code_embeddings`)
+
+### Limitations
+
+This tool requires GPU/CPU resources to run the embedding model (`Qwen/Qwen3-Embedding-4B`). The pre-built index is downloaded from the Hub by default, which requires an internet connection on first use.
+
+Results are suggestions based on code similarity and should be manually reviewed before modularization. High similarity scores don't guarantee perfect compatibility.