Provide better APIs for working with models that have many inputs / outputs

[robertknight]

The current APIs for running models in RTen are annoying to use if a model has many inputs or outputs. The current APIs require a list of (NodeId, Input) tuples to be passed for the input, and a list of NodeIds of outputs. It would however be more ergonomic if:

• The inputs/outputs could be specified as names, without requiring the NodeId lookup step first
• There was a convenient way to build up the inputs as a hash map
• Outputs could easily be retrieved by name rather than index

This issue originally came up when porting the Tacotron2 TTS demo from https://github.com/xd009642/xd-tts/ to RTen. See page 38 of https://github.com/xd009642/xd-tts/blob/master/slides/slides.pdf for an example of why this matters.