Refactor code

src/fsrs/__init__.py

@@ -5,6 +5,7 @@
 Py-FSRS is the official Python implementation of the FSRS scheduler algorithm, which can be used to develop spaced repetition systems.
 """
 
-from .fsrs import Scheduler, Card, Rating, ReviewLog, State
+from .models import Card, Rating, ReviewLog, State
+from .scheduler import Scheduler
 
 __all__ = ["Scheduler", "Card", "Rating", "ReviewLog", "State"]