Remove mamba-ssm package

[tlrmchlsmth]

Purpose

The packages mamba-ssm and causal-conv-1d are not used by vLLM but are used to compare against the huggingface transformer baselines in tests.

In particular, those packages are need in order to run PLaMo2 at all. The model definition for PLaMo2 is at https://huggingface.co/pfnet/plamo-2-1b/blob/main/modeling_plamo.py, rather than being in huggingface transformers.

This PR removes the packages to avoid pain points when upgrading PyTorch or CUDA.

Test Plan

Test Result

[gemini-code-assist]

Code Review

This pull request removes the mamba-ssm package and its related causal-conv-1d dependency, which were only used for testing the PLaMo2 model against its Hugging Face implementation. The changes correctly remove the dependencies from requirements/test.in, the Dockerfile, and documentation.

However, I've identified a critical issue in tests/models/language/generation/test_hybrid.py where the PLaMo2 model (pfnet/plamo-2-1b) is completely removed from the test suite, instead of just skipping the comparison with the Hugging Face baseline. I've provided a suggestion to fix this to ensure the model remains tested by vLLM.

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[DarkLight1337]

Some other tests still require mamba-ssm. It's just that causal-conv1d is not required

[tlrmchlsmth]

Some other tests still require mamba-ssm. It's just that causal-conv1d is not required

I see, you're right - there are both numeric issues and also seeing this:

[2025-08-07T01:50:59Z]         if self.use_fast_kernels:
--
  | [2025-08-07T01:50:59Z]             if not is_fast_path_available or "cuda" not in self.x_proj.weight.device.type:
  | [2025-08-07T01:50:59Z] >               raise ValueError(
  | [2025-08-07T01:50:59Z]                     "Fast Mamba kernels are not available. Make sure to they are installed and that the mamba module is on a CUDA device"
  | [2025-08-07T01:50:59Z]                 )
  | [2025-08-07T01:50:59Z] E               ValueError: Fast Mamba kernels are not available. Make sure to they are installed and that the mamba module is on a CUDA device
  | [2025-08-07T01:50:59Z]
  | [2025-08-07T01:50:59Z] /usr/local/lib/python3.12/dist-packages/transformers/models/jamba/modeling_jamba.py:826: ValueError

[Alnusjaponica]

As mentioned earlier #20047 (comment), some tests run pfnet/palmo-2-1b inference with transformers implementation, which implicitly depends on both mamba-ssm and causal-conv1d. Sorry for the inconvenience.
Maybe we can avoid this dependency by hardcoding the inference results. Is it acceptable for the unit tests?

[tdoublep]

Can't we just remove these as vLLM dependencies but install them inside the test image? This is what we are doing now for the causal_conv_1d package.

Related topic: the hybrid tests are completely messed up right now due to some CUDA version mismatch issue. This would also be solved if we install mamba_ssm inside the test container but in the "right way" (--no-build-isolation).

[tdoublep]

This is what I propose as a different solution to this problem:
#22541

[DarkLight1337]

Closing in favor of #22541