Make pyannote.audio optional (still mandatory to run default pipeline)

[juanmc2005]

This PR addresses issue #43

Changelog

• Replace --gpu with --cpu. Runs on GPU by default.
• Add SegmentationModel and EmbeddingModel as base classes for custom pre-trained models (into a new models module)
  • pyannote models can be loaded using SegmentationModel.from_pyannote("pyannote/segmentation") if pyannote.audio is installed
• Run diart.stream and diart.benchmark on GPU by default
• Add audio module and AudioLoader to load audio from a file
• Merge ChunkLoader into AudioLoader
• Rename FramewiseModel to SpeakerSegmentation
• Rename ChunkwiseModel to SpeakerEmbedding
• Rewrite Binarize so it does not depend on pyannote.audio
• Unify pipeline configuration so that PipelineConfig is the only source of truth
• Add some docstrings
• Update dependencies