model: support MiMo-V2-Flash (#18328)

model: support MiMo-V2-Flash (#18328)

* mimov2: convert ok

* rename mimov2 --> mimo2

* fix conversion

* runnable not incorrect

* use sink

* add_sliding_window_pattern

* add swa and per-layer n_head_kv

* correct params

* somewhat working

* correct gating func

* nits

* mimo2: wire RMS eps + MoE bias + converter guards

* add co-author

Co-authored-by: Aaryan-Kapoor <redacted>
* use add_rope_freq_base_swa

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Co-authored-by: Aaryan Kapoor <redacted>
Co-authored-by: Aaryan-Kapoor <redacted>