Unify sleap-track and python -m sleap.nn.tracking.

• Option 1: If no models found in policies AND the input data is a serialized Labels file with predictions, then pass parameters over to sleap.nn.tracking.retrack

• Option 2: Implement sleap.nn.tracking.retrack equivalent functionality in sleap.nn.inference.Predictor

The latter is preferable and could be accomplished by loading existing predicted instances instead of calling predict_chunk here:
https://github.com/murthylab/sleap/blob/4b4f773ab7cf9323a591ba308deeb2e10db64be6/sleap/nn/inference.py#L83-L85