Lazy: Simplify method signature; switch to AbstractModel

miplearn/solvers/abstract.py

@@ -3,7 +3,7 @@
 #  Released under the modified BSD license. See COPYING.md for more details.
 
 from abc import ABC, abstractmethod
-from typing import Optional, Dict
+from typing import Optional, Dict, Callable
 
 import numpy as np
 
@@ -16,6 +16,10 @@ class AbstractModel(ABC):
     _supports_node_count = False
     _supports_solution_pool = False
 
+    def __init__(self) -> None:
+        self.lazy_enforce: Optional[Callable] = None
+        self.lazy_separate: Optional[Callable] = None
+
     @abstractmethod
     def add_constrs(
         self,