Add mamba-minimal

- Add stateless forward impl.
  - Efficient for training (but training is not yet implemented).
  - Input requires the entire sequence, and requires no state cache.
  - Generates one output for each input sequence.
- Add stateful forward impl.
  - Efficient for inference.
  - Input requires the last single sequence point, and requires the last state cache.
  - Generates a single output referring to the last input.

Author: swfsql