[Feature]: Support `Phi4Flash` model in V1

[tdoublep]

🚀 The feature, motivation and pitch

We will drop support for V0 in the very near future, and if we want to continue supporting this model in vLLM, it needs to be ported to V1.

The main work here is in:

1. Adapting the custom Mamba layer to match how V1 manages the mamba state. The MambaMixer, MambaMixer2, MinimaxLinearAttention and ShortConv can all be used as a reference for how to do this.
2. Porting the differential attention backend to V1.

Alternatives

Drop model support in vLLM

Additional context

No response

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[gigit0000]

I’d be glad to take it if first-timers would rather pass.

[aditchawdhary]

Can I take this? I am a first-ish timer

[gigit0000]

I'll step aside.