Fix/3170 dense blockscaled sm12x (#3180)

## 📌 Description

  After #3113 extended the b12x decorator to SM121, calling `mm_fp4(...,
  backend="b12x")` on sm_121 still trips the dispatch-side check in
  `dense_blockscaled_gemm_sm120_b12x.py:1591`:

  ```
  ValueError: dense_gemm launch only supports sm_120, got sm_121
  ```

SM120 and SM121 share the same 12.x spec (MMA atoms, SMEM), so the same
kernel applies. Allow sm_121 through the check. The error message is
updated to match.

  Other call sites in this file (`sm_version="sm_120"` at line 1875,
`get_smem_capacity_in_bytes("sm_120")` at lines 122/1461) are left alone
— they work for both arches.

  ## 🔍 Related Issues

  Refs #3170 (Action Item 4). Follows up on #3113.

  ## 🚀 Pull Request Checklist

  ### ✅ Pre-commit Checks

- [x] I have installed `pre-commit` by running `pip install pre-commit`
(or used your preferred method).
  - [x] I have installed the hooks with `pre-commit install`.
- [x] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

  ## 🧪 Tests

  - [ ] Tests have been added or updated as needed.
  - [ ] All tests are passing (`unittest`, etc.).

No new tests; no SM121 hardware locally. Existing sm_120 path unchanged.

  ## Reviewer Notes

Same audit cleanup batch as #3173 / #3174 / #3175.

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## Summary by CodeRabbit

* **Improvements**
* Expanded GPU architecture support to include additional SM12x devices
(sm_120 and sm_121).

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