Build model (#192)

* move build files to separate directory

* typo

Author: mikekgfb

builder.py renamed to build/builder.py

@@ -21,7 +21,7 @@
 from typing import Union, Optional
 
 from sentencepiece import SentencePieceProcessor
-from model import Transformer
+from build.model import Transformer
 
 
 @dataclass