[CLI] Clean up and improvements to diffusers CLI

[DN6]

What does this PR do?

Few things that were missed in the initial implementation.

In this PR:

• Write modular_model_index.json when saving a custom block. This allows ModularPipeline.from_pretrained to load and run custom blocks as pipelines.
• Add auto CPU offload support for modular pipelines
• Clean up output saving to handle multimodal output e.g. LTX video frames + audio, and batched video.
• Add imageio and imageio-ffmpeg to required remote deps so that we can save videos with remote inference.

Fixes # (issue)

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