[pytorch] Add error when PyTorch used with Python 2 (pytorch#36151)

Summary:
Pull Request resolved: pytorch#36151

Python 2 has reached end-of-life and is no longer supported by PyTorch. To avoid confusing behavior when trying to use PyTorch with Python 2, detect this case early and fail with a clear message.  This commit covers `import torch` only and not C++  for now.

Test Plan: waitforsandcastle

Reviewed By: dreiss

Differential Revision: D20894381

fbshipit-source-id: a1073b7a648e07cf10cda5a99a2cf4eee5a89230

torch/__init__.py

@@ -13,6 +13,10 @@
 import sys
 import platform
 import ctypes
+
+if sys.version_info < (3,):
+    raise Exception("Python 2 has reached end-of-life and is no longer supported by PyTorch.")
+
 from ._utils import _import_dotted_name
 from ._utils_internal import get_file_path, prepare_multiprocessing_environment, \
     USE_RTLD_GLOBAL_WITH_LIBTORCH