refactor(mixin): initialize bm with brainpy.version2.math if None

Author: oujago

brainpy/mixin.py

@@ -47,6 +47,10 @@
 
 def _get_bm():
     global bm
+    if bm is None:
+        from brainpy.version2 import math
+        bm = math
+    return bm
 
 
 class AlignPost(brainstate.mixin.Mixin):