Add a base_model_paths property to _DiffusionServingModels.

[sjuxax]

Purpose

Get a 500 Error without this.

Test Plan

Test committed to verify that _get_engine_and_models returns correctly when there's a single diffusion stage.

Test Result

Test good, 500 error gone.

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[alex-jw-brooks]

I think there is already a fix open for this one! #1771

[lishunyang12]

LGTM — matches how upstream OpenAIServingModels exposes base_model_paths.

[lishunyang12]

I think there is already a fix open for this one! #1771

oh @sjuxax Please check if it is duplicated work.