[Feature]: Support `Plamo2` 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 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.

Alternatives

Drop model support in vLLM.

Additional context

No response

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[tdoublep]

It sounds like @nopperl has already got this working. Looking forward to review the PR.

[hmellor]

Plamo2 also has a fundamental issue aht it inherits from PreTrainedModel and has clashing class method names. This must also be resolved for Plamo2 to work on V0 or V1