Support Gemma 4 12B Unified in the JAX package

Hi Gemma team,

Thanks for maintaining this package. I am opening this as a support / enhancement question rather than a bug report; apologies if I missed an existing supported path or a duplicate issue.

I am trying to use the released Gemma 4 12B Unified model from the JAX `gemma` package. In my checkout of current `main` (`05652d59d546d3822b3dfc1f60133cabfe265cbb`), Gemma 4 support appears to expose these model classes:

- `gm.nn.Gemma4_E2B`
- `gm.nn.Gemma4_E4B`
- `gm.nn.Gemma4_26B_A4B`
- `gm.nn.Gemma4_31B`

and matching checkpoint enum entries:

- `CheckpointPath.GEMMA4_E2B_*`
- `CheckpointPath.GEMMA4_E4B_*`
- `CheckpointPath.GEMMA4_26B_A4B_*`
- `CheckpointPath.GEMMA4_31B_*`

I did not find a `Gemma4_12B`, `GEMMA4_12B_*`, `gemma4-12b`, or `12B Unified` equivalent in the JAX package surface.

The public model appears to be available as:

- https://huggingface.co/google/gemma-4-12B
- https://huggingface.co/google/gemma-4-12B-it
- https://ai.google.dev/gemma/docs/core/model_card_4

Would you be open to adding first-class JAX support for this variant, or documenting the intended load path if it already exists?

Specifically, it would be helpful to know:

1. Is Gemma 4 12B Unified support planned for this repo?
2. Should users expect a new `gm.nn.Gemma4_12B` class and `CheckpointPath.GEMMA4_12B_*` enum entries, or a different naming/API?
3. Is there an interim way to load the 12B Unified Hugging Face checkpoint with this package?

Why I am asking: Gemma 4 12B Unified seems to have different architecture/runtime assumptions from the currently exposed Gemma 4 variants, and downstream projects are already tracking support issues for it. A canonical JAX path or doc note here would help users avoid guessing.

Thanks!

Start by inspecting the existing gm.nn Gemma 4 classes and CheckpointPath.GEMMA4_* entries mentioned in the issue, then compare them with the released Gemma 4 12B checkpoints. Done means either first-class loading support is implemented and tested for the 12B Unified variants, or the intended Hugging Face load path is documented.

Written by the indexing model from the issue text.