[BugFix] Fix minimax m2 model rotary_dim

[rogeryoungh]

Purpose

After #29966, get_rope always reads partial_rotary_factor from the configuration and performs the multiplication again, even if rotary_dim has already been scaled. This leads to the factor being applied repeatedly, incorrectly reducing the effective rotational dimension.

Another way to fix #30349.

Test Result

lm_eval --model local-completions \
    --model_args base_url=http://localhost:10086/v1/completions,tokenizer=/model,model=/model \
    --tasks gsm8k_cot  \
    --batch_size 128 \
    --num_fewshot 5 

local-completions (base_url=http://localhost:10086/v1/completions,tokenizer=/model,model=/model), gen_kwargs: (None), limit: None, num_fewshot: 5, batch_size: 128
|  Tasks  |Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|---------|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k_cot|      3|flexible-extract|     5|exact_match|↑  |0.9219|±  |0.0074|
|         |       |strict-match    |     5|exact_match|↑  |0.9098|±  |0.0079|

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[chatgpt-codex-connector]

Codex usage limits have been reached for code reviews. Please check with the admins of this repo to increase the limits by adding credits.

[gemini-code-assist]

Code Review

This pull request correctly addresses a bug in the minimax_m2 model where the rotary dimension was being scaled twice. The issue stemmed from passing an already-scaled rotary_dim to the get_rope function, which would then apply the partial_rotary_factor again. The proposed fix of passing self.head_dim as the rotary_dim to get_rope is the correct approach, as it provides the unscaled base dimension, allowing get_rope to perform the scaling correctly. The change is logical, minimal, and effectively resolves the described problem.

[hmellor]

LGTM! Thanks for the fix