Format versioning for model artifacts

[Waino]

A version number should be stored in the model save files (checkpoints).
Whenever a code change affects the format of the save files, the version should be incremented.
When loading a save file with a version that is older than the current version:

• Log a warning including the version in the save file.
• Perhaps attempt to transform the format to be compatible. Such compatibility code becomes clearer to read and easier to remove once obsolete, because it is enclosed in a condition on the save format version.
• If recovery is impossible with the current code, throw an exception. It is easy to roll back to a commit with the appropriate format version, to allow decoding or continued training.