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

setup.py

@@ -17,8 +17,8 @@
     zip_safe=False,
     install_requires=[
         "numpy>=1.14.2",
-        "torch>=1.5.1",
-        "torchvision>=0.6.1",
+        "torch==1.7.1",
+        "torchvision==0.8.2",
         "tensorboardX>=1.7",
         "tqdm>=4.19.9",
         "matplotlib>=2.1.0",