[BugFix] Fix minimax m2 model partial_rotary_factor

[rogeryoungh]

Purpose

In #30384, I fixed an inference issue with MiniMax-M2, but this caused some community-contributed quantized models to fail(#30445), some of which were forked before I added partial_rotary_factor. This update provides compatibility with those community-contributed quantized models.

Test Plan

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

Test Result

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|↑  |0.9227|±  |0.0074|
|         |       |strict-match    |     5|exact_match|↑  |0.9105|±  |0.0079|

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• 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
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[gemini-code-assist]

Code Review

This pull request aims to fix a compatibility issue with older quantized models by conditionally setting the rotary_dim. While the overall logic seems correct, I've identified a critical issue in the implementation that could lead to an UnboundLocalError at runtime. My review includes a suggested fix for this issue.