Dev audio loader

[leichtrhino]

This PR includes:

• introducing following AudioLoader operator
  • slice operation (e.g. loader[:len(loader)//2].)
  • Combiner to make new AudioLoder, combination of some AudioLoaders.
  • split_loader to split a loader into training fold and validation fold.
• adding offset and duration to input signature of AudioLoader.load_audio().
• change input signature of VoxCelebLoader.__init__() (use Combiner to do VoxCelebLoader(dev_path, test_path).)
• delete VoxCeleb2Loader (was replaced with VoxCelebLoader.)
• (internal) changed way to load audio from compressed files.