Add info about property wrappers to porting guide

[DePasqualeOrg]

Please revise this if necessary. I generated the section on property wrappers based on the existing models in this repo.

[xlab]

I recommend adding a tricky case #340 (comment)
when a param doesn't exist during load, and must be computed. You want to differentiate a field for that, and that must not be confused with self._field.wrappedValue.

Example:

 @ParameterInfo(key: "correct_output_scale") var correctOutputScale: MLXArray
  private let _routerInputScale: MLXArray

// yes:
self._correctOutputScale.wrappedValue = MLXArray(...

// but:
self._routerInputScale = MLXArray(...

[davidkoski]

Looks great, thank you!