[llvm][NFC] ML Policies: changed the saved_model protobuf to text

Also compacted the checkpoints (variables) to one file (plus the index).

This reduces the binary model files to just the variables and their
index. The index is very small. The variables are serialized float
arrays. When updated through training, the changes are very likely
unlocalized, so there's very little value in them being anything else
than binary.