Add inference pipeline base class to streamline composed pipelines

[SolitaryThinker]

Summary

• add a dedicated InferencePipeline layer that handles inference-specific initialization
• update all inference and preprocess pipelines to inherit from the new base and refresh documentation
• move training-specific setup into TrainingPipeline.setup_pipeline to keep ComposedPipelineBase shared

Testing

• python -m compileall fastvideo/pipelines fastvideo/training docs/source

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Codex Task