text: Title: Add save\load primitives to Hummingbird
  Body: With the current design, saving and loading models has a couple of drawbacks (listed in #321). To overcome these we can add `save` and `load` primitives to HB.
  
  A possibility is to have `save` and `load` as part of the package (as for `torch.save` and `onnx.save`) so that users can do `hummingbird.ml.save` for instance.
  
  We should also have an option to save only the base model and not the container. Something like a `save_native_model_only=False` flag.