[Bugfix] Fix precision issues in moe_mlp (vllm-ascend main)

[Clorist33]

What this PR does / why we need it?

Use group_list[0] to replace group_diff[0] in function "cumsum_group_list" (moe_mlp.py).
The purpose is to modify it to the correct logic of converting cumsum to count.

Does this PR introduce any user-facing change?

No

• vLLM version: v0.12.0
• vLLM main: vllm-project/vllm@ad32e3e

[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in the cumsum_group_list function by changing group_diff[0] to group_list[0]. This ensures the logic for converting a cumulative sum list to a difference list is correct. The change is accurate and addresses the intended issue. I've also added a comment to handle a potential IndexError on an empty tensor, which would improve the robustness of the function.

[gemini-code-assist]

There's a potential IndexError here if group_list is an empty tensor. Accessing group_list[0] on line 49 would cause a crash. It's good practice to handle this edge case, for example by checking if the tensor is empty before proceeding.

Suggested change

	if src_list_type == 0 and dst_list_type == 1:
	group_diff = torch.diff(group_list)
	new_group = torch.cat([group_diff[0].unsqueeze(0), group_diff], dim=0)
	new_group = torch.cat([group_list[0].unsqueeze(0), group_diff], dim=0)
	return new_group
	if src_list_type == 0 and dst_list_type == 1:
	if not group_list.numel():
	return group_list
	group_diff = torch.diff(group_list)
	new_group = torch.cat([group_list[0].unsqueeze(0), group_diff], dim=0)
	return new_group

[github-actions]

👋 Hi! Thank you for contributing to the vLLM Ascend project. The following points will speed up your PR merge:‌‌

• A PR should do only one thing, smaller PRs enable faster reviews.
• Every PR should include unit tests and end-to-end tests ‌to ensure it works and is not broken by other future PRs.
• Write the commit message by fulfilling the PR description to help reviewer and future developers understand.

If CI fails, you can run linting and testing checks locally according Contributing and Testing.