LFM2 (#39340)

* [modeling][lfm2] LFM2 model on 4.53.0 interface

* [configuration] hook in LFM2 keys

* [modeling][lfm2] update modeling interface for 4.53.1

* [modeling][lfm2] apply mask to hidden conv states

* [misc] ruff format/lint

* [modeling][lfm2] minor: NotImplemented legacy cache conversion

* Create lfm2.md

* create nice modular

* style

* Update modeling_auto.py

* clean and start adding tests

* style

* Update test_modeling_lfm2.py

* Update __init__.py

* small test model size

* config

* small fix

* fix

* remove useless config attrs -> block_dim and conv_dim are hiden_size

* fix prepare inputs

* fix config

* test

* typo

* skip tests accordingly

* config docstrings

* add doc to .md

* skip config docstring check

---------

Co-authored-by: Maxime Labonne <81252890+mlabonne@users.noreply.github.com>
Co-authored-by: Cyril Vallez <cyril.vallez@gmail.com>

docs/source/en/model_doc/lfm2.md

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+<div class="flex flex-wrap space-x-1">
+<img alt="PyTorch" src="https://img.shields.io/badge/PyTorch-DE3412?style=flat&logo=pytorch&logoColor=white">
+</div>
+
+# LFM2
+
+## Overview
+
+[LFM2](https://www.liquid.ai/blog/liquid-foundation-models-v2-our-second-series-of-generative-ai-models) represents a new generation of Liquid Foundation Models developed by [Liquid AI](https://liquid.ai/), specifically designed for edge AI and on-device deployment. 
+
+The models are available in three sizes (350M, 700M, and 1.2B parameters) and are engineered to run efficiently on CPU, GPU, and NPU hardware, making them particularly well-suited for applications requiring low latency, offline operation, and privacy.
+
+## Architecture
+
+The architecture consists of 16 blocks total: 10 double-gated short-range convolution blocks and 6 blocks of grouped query attention. This design stems from the concept of dynamical systems, where linear operations are modulated by input-dependent gates, allowing for "liquid" dynamics that can adapt in real-time. The short convolutions are particularly optimized for embedded SoC CPUs, making them ideal for devices that require fast, local inference without relying on cloud connectivity.
+
+The key architectural innovation of LFM2 lies in its systematic approach to balancing quality, latency, and memory efficiency through our STAR neural architecture search engine. Using STAR, Liquid AI optimized the models for real-world performance on embedded hardware, measuring actual peak memory usage and inference speed on Qualcomm Snapdragon processors. This results in models that achieve 2x faster decode and prefill performance compared to similar-sized models, while maintaining superior benchmark performance across knowledge, mathematics, instruction following, and multilingual tasks.
+
+## Example
+
+The following example shows how to generate an answer using the `AutoModelForCausalLM` class.
+
+```python
+from transformers import AutoModelForCausalLM, AutoTokenizer
+
+# Load model and tokenizer
+model_id = "LiquidAI/LFM2-1.2B"
+model = AutoModelForCausalLM.from_pretrained(
+    model_id,
+    device_map="auto",
+    torch_dtype="bfloat16",
+)
+tokenizer = AutoTokenizer.from_pretrained(model_id)
+
+# Generate answer
+prompt = "What is C. elegans?"
+input_ids = tokenizer.apply_chat_template(
+    [{"role": "user", "content": prompt}],
+    add_generation_prompt=True,
+    return_tensors="pt",
+    tokenize=True,
+)
+
+output = model.generate(
+    input_ids,
+    do_sample=True,
+    temperature=0.3,
+    min_p=0.15,
+    repetition_penalty=1.05,
+    max_new_tokens=512,
+)
+
+print(tokenizer.decode(output[0], skip_special_tokens=False))
+```
+
+## Lfm2Config
+
+[[autodoc]] Lfm2Config
+
+## Lfm2Model
+
+[[autodoc]] Lfm2Model
+    - forward
+
+## Lfm2ForCausalLM
+
+[[autodoc]] Lfm2ForCausalLM
+    - forward