Fix deeplift mypy error (#1459)

Summary:

Currently, Captum OSS tests are failing due to mypy failures (likely from new version) in DeepLift test cases. Adds fix for type failure caused by different signature between DeepLift and DeepLiftShap.

Differential Revision: D67538043

Author: Vivek Miglani

tests/attr/test_deeplift_classification.py

@@ -2,7 +2,7 @@
 
 # pyre-unsafe
 
-from typing import Union
+from typing import TypeVar, Union
 
 import torch
 from captum._utils.typing import TargetType
@@ -21,6 +21,8 @@
 from torch import Tensor
 from torch.nn import Module
 
+DeepLiftAttrMethod = TypeVar("DeepLiftAttrMethod", DeepLift, DeepLiftShap)
+
 
 class Test(BaseTest):
     def test_sigmoid_classification(self) -> None:
@@ -155,7 +157,7 @@ def test_convnet_with_maxpool1d_large_baselines(self) -> None:
     def softmax_classification(
         self,
         model: Module,
-        attr_method: Union[DeepLift, DeepLiftShap],
+        attr_method: DeepLiftAttrMethod,
         input: Tensor,
         baselines: Union[float, int, Tensor],
         target: TargetType,