moe_router_load_balancing_type has inconsistent type declarations across different code locations

[alkaideemo]

Checklist / 检查清单

• I have searched existing issues, and this is a new bug report. / 我已经搜索过现有的 issues，确认这是一个新的 bug report。

Bug Description / Bug 描述

moe_router_load_balancing_type has inconsistent type declarations across different code locations

How to Reproduce / 如何复现

In megatron_args, list

ms-swift/swift/megatron/arguments/megatron_args.py

Line 511 in f0aecc0

moe_router_load_balancing_type: Optional[List[str]] = None

In model_config, string

ms-swift/swift/megatron/model/model_config.py

Lines 175 to 176 in f0aecc0

	moe_router_load_balancing_type: Literal['aux_loss', 'seq_aux_loss', 'global_aux_loss', 'sinkhorn',
	'none'] = 'aux_loss'

Additional Information / 补充信息

No response

[yuchenwang3]

Confirmed on current main, and it's not just cosmetic — the inconsistency breaks the documented "disable" path.

Three locations disagree:

• swift/megatron/arguments/megatron_args.py: Optional[List[str]]
• mcore_bridge ModelConfig: Literal['aux_loss', 'seq_aux_loss', 'global_aux_loss', 'sinkhorn', 'none'] (single string)
• swift/megatron/trainers/base.py: compared as a string (config.moe_router_load_balancing_type == 'aux_loss', ...)

Because the arg is a List[str], --moe_router_load_balancing_type none (the documented way to turn load balancing off) parses to ['none'], which then never equals any of the string branches downstream → load balancing isn't actually disabled and the metric logging in base.py is skipped.

Opened #9600 to make the arg a single-string Literal matching ModelConfig (keeping Optional/None for the "inherit from config.json" default). Megatron-core itself types this as Union[str, List[str]], but mcore_bridge.ModelConfig (what ms-swift feeds) only exposes the single-string form and there's no list-handling code in ms-swift, so aligning to the string is the consistent fix; exposing the list form would be a separate feature.