Add convenience Functions to save/load quantized Model to/from Disk

[marcnnn]

Since loading Quantized models from HF is not possible, jet.

I was searching for an easy way to safe models after quantization, as easy as loading them from HF.

And then a function to load the file again.

[jonatanklosko]

This probably belongs more to Axon than Bumblebee, since we need a way to store %Axon.ModelState{}. For the model itself, maybe there should be a way to quantize the model only, so it can be replicated without altering the params (so we can still build the model, instead of storing it). cc @seanmor5