import error fix

Author: HojeChun

nff/io/chgnet.py

@@ -12,6 +12,7 @@
 from nff.io import AtomsBatch
 from nff.utils.cuda import batch_detach, detach
 from chgnet.graph import CrystalGraph, CrystalGraphConverter
+from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from collections.abc import Sequence