What does this PR do?

Hello, this PR aims to add the MiMo-V2-Flash model to the Transformers library
Fixes #42954

MiMo-V2 is "The last of the OSS SOTAs" that isn't natively supported by the Transformers library (besides Kimi), so I hope we can make this work.

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Only pinging HF engineers who were involved in the PRs mentioned below:

Modular Sensei @vasqu , I took your comments from the other PR, to find the best candidates to inherit from. Although see #45144 (comment) for some important points.

@ArthurZucker for gpt-oss similarity and text model

For reference, there were already 2 previous PRs (1 closed and 1 non modular/abandoned):

• implemented mimo model #42995
• Add mimo v2 flash #43020

Afaik, MiMo in the codebase is a bit novel as it's the only hybrid SWA that combines both dual theta RoPE and dual head
dims + attn sinks (like gpt-oss). Most similar models are either one or the other, but not both.
So modularity wasn't easy mode pass, pass, pass.

I added some verbose NOTES for context/choices, and make the review easier. I'll obviously remove these once things are settled.

Additional useful info

(copy pasta from my own readme impl)

• First layer is fully dense (GA+FFN, not MoE)
• No shared experts in MoE
• SWA and GA layers have different RoPE theta bases
• SWA and GA layers have a different number of KV groups (GQA)
• Values head dim is decoupled from QK head dim
• Values are rescaled by sqrt(hidden_size / (num_attention_heads * v_head_dim))
• Attention sink is only applied for SWA layers
• Partial RoPE (rotating first 33% of the head dim)
• Triple dtype with 1 quant