Fix DeepSpeedInferenceConfig crash on bool moe backward-compat value

[winklemad]

DeepSpeedInferenceConfig.moe is typed Union[bool, DeepSpeedMoEConfig], and the moe_backward_compat validator explicitly handles the bool form (the old init_inference(..., moe=<bool>) interface). But it builds DeepSpeedMoEConfig(moe=field_value) — and DeepSpeedMoEConfig has no moe field (it has enabled). Since DeepSpeedConfigModel sets extra="forbid", every bool moe raises a ValidationError instead of building the config:

>>> from deepspeed.inference.config import DeepSpeedInferenceConfig
>>> DeepSpeedInferenceConfig(moe=True)
pydantic_core._pydantic_core.ValidationError: 1 validation error for DeepSpeedMoEConfig
moe
  Extra inputs are not permitted [type=extra_forbidden, ...]

Both moe=True and moe=False crash, so the documented backward-compat path is unusable. The fix passes enabled=field_value (the actual field) so the bool is turned into a DeepSpeedMoEConfig as intended.

The line dates back to #2516 and survived the pydantic-v2 migration (#5167) unchanged.

Added a regression test (test_moe_backward_compat_bool) asserting the bool form builds a DeepSpeedMoEConfig with the matching enabled. yapf and flake8 clean.

[tohtana]

Great catch, thank you @winklemad!