Make loading pre-trained TAN a one-liner

[ajratner]

Right now we have to load in:

1. The TAN checkpoint
2. Config file
3. TFs
4. dims

(4) should be merged into (2); (3) should also be merged into (2) (see e.g. scuba repo); and then everything should be put into one folder so everything can be auto-loaded with one path

[henryre]

Just to clarify from our discussion: you should be able to pass in either a TAN object or a string as the first argument to TANDAImageDataGenerator