add missing import

Author: janfb

sbi/neural_nets/estimators/base.py

@@ -2,7 +2,7 @@
 # under the Apache License Version 2.0, see <https://www.apache.org/licenses/>
 
 from abc import ABC, abstractmethod
-from typing import Optional, Protocol, Tuple, TypeVar
+from typing import Optional, Protocol, Tuple, TypeVar, Union
 
 import torch
 from torch import Tensor, nn