[Model] Remove the unnecessary dtype conversion in MiniCPM

[gcanlin]

Purpose

In the past, vllm-ascend has to add the patch for the unsupported dtype conversion in MiniCPMAttention forward. For removing the patch, we upstream the change into vLLM. For now, the conversion would be unnecessary for any platforms because the kernels handle precision internally.

If necessary, I will give a acc test for MiniCPM.

Test Plan

See vllm-project/vllm-ascend#5975.

Test Result

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[gemini-code-assist]

Code Review

This pull request removes an explicit dtype conversion to float32 in MiniCPMAttention before applying rotary embeddings. The rationale is that the underlying kernels now handle precision internally. While this is likely true for optimized custom kernels, I've raised a concern about the PyTorch-native fallback path (forward_native), which could suffer from precision loss without this explicit casting. I've recommended restoring the casting to ensure numerical stability across all execution paths.

[gcanlin]

@Isotr0py Hi :) Could you please help take a look?

[Isotr0py]

LGTM, but would be better to have some acc benchmark results in case.

[gcanlin]

@Isotr0py Thanks! Here is the comparison between before and after this change. It didn't cause the acc regression.

Command:

vllm serve openbmb/MiniCPM-2B-sft-bf16 --trust-remote-code --max_num_batched_tokens 65536
evalscope eval  --model openbmb/MiniCPM-2B-sft-bf16  --api-url http://localhost:8000/v1  --api-key EMPTY  --eval-type server  --datasets gsm8k

Before:

+---------------------+-----------+----------+----------+-------+---------+---------+
| Model               | Dataset   | Metric   | Subset   |   Num |   Score | Cat.0   |
+=====================+===========+==========+==========+=======+=========+=========+
| MiniCPM-2B-sft-bf16 | gsm8k     | mean_acc | main     |  1319 |  0.4428 | default |
+---------------------+-----------+----------+----------+-------+---------+---------+

After:

+---------------------+-----------+----------+----------+-------+---------+---------+
| Model               | Dataset   | Metric   | Subset   |   Num |   Score | Cat.0   |
+=====================+===========+==========+==========+=======+=========+=========+
| MiniCPM-2B-sft-bf16 | gsm8k     | mean_acc | main     |  1319 |   0.442 | default |
+---------------------+-----------+----------+----------+-------+---------+---------+