Freeze PyTorch to ver 1.7.1

Temporarily freeze PyTorch to version 1.7.1, since 1.8 have some unsolved issues - i.e. [#53485](pytorch/pytorch#53485)

Author: Hananel-Hazan

requirements.txt

@@ -2,8 +2,8 @@ foolbox
 scipy>=1.1.0
 numpy>=1.14.2
 cython>=0.28.5
-torch>=1.5.1
-torchvision>=0.6.1
+torch==1.7.1
+torchvision==0.8.2
 tqdm>=4.19.9
 setuptools>=39.0.1
 matplotlib>=2.1.0