feat: lazy-load multimodal package.

[pengcheng888]

Summary

问题：在bw1100上，torch版本是2.11，在一些音视频的包有问题，导致运行非多模态的模型环境报错。

解决方式：包放在全局的位置，默认都加载。但文本模型不需要引入音视频的包。所以延迟import。

该pr只调整了import的位置，从全局移动到了代码中。理论上不会影响任何功能。

bw1000和bw1100编译都正常，都可以做端到端graph的推理。

bw1000

bw1100

Motivation

Closes #

Type of Change

• feat — new feature / new model
• fix — bug fix
• perf — performance improvement (no behavioral change)
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• Breaking change

Test Results of Involved Models on Supported Platforms (Please attach screenshots)

Benchmark / Performance Impact

Notes for Reviewers

CI / ChatOps

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