TFLite Model Maker 0.3.2

• Minor fix: Move librosa to an optional library for audio task.
• Update readme with links to new colabs and API reference.

TFLite Model Maker 0.3.1

• Polish document showing in https://www.tensorflow.org/lite/api_docs/python/tflite_model_maker.
• Add some 0.2.x APIs for backward compatibility.
• Export TFLite Model Maker public APIs, and adapt colabs as demo code.
• Add three new tasks: Object Detection, Audio Classification and Recommendation.

TFLite Model Maker 0.2.4

• Update stable tensorflow version in requirements.txt.
• Support exporting to TFJS models.
• Refine Code: refactor the gen_dataset internal API in DataLoader and remove redundant methods.
• Split requirements to stable version and nightly version.