Add warning for building PyTorch using Python 2.7 on Windows (pytorch…

…#10247)

Summary:
Fixes pytorch#9232.
Pull Request resolved: pytorch#10247

Differential Revision: D9178257

Pulled By: SsnL

fbshipit-source-id: cc553335a5a918b6d77fe1064460cb66114859ca

README.md

@@ -148,7 +148,9 @@ If you want to compile with CUDA support, install
 If you want to disable CUDA support, export environment variable `NO_CUDA=1`.
 Other potentially useful environment variables may be found in `setup.py`.
 
-If you want to build on Windows, Visual Studio 2017 and NVTX are also needed.
+If you want to build on Windows, Visual Studio 2017 14.11 toolset and NVTX are also needed.
+Especially, for CUDA 8 build on Windows, there will be an additional requirement for VS 2015 Update 3 and a patch for it.
+The details of the patch can be found out [here](https://support.microsoft.com/en-gb/help/4020481/fix-link-exe-crashes-with-a-fatal-lnk1000-error-when-you-use-wholearch).
 
 #### Install optional dependencies
 
@@ -196,9 +198,10 @@ On Windows
 set "VS150COMNTOOLS=C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Auxiliary\Build"
 set CMAKE_GENERATOR=Visual Studio 15 2017 Win64
 set DISTUTILS_USE_SDK=1
-REM The following line is needed for Python 2.7, but the support for it is very experimental.
+REM The following two lines are needed for Python 2.7, but the support for it is very experimental.
 set MSSdk=1
-REM As for CUDA 8, VS2015 Update 2 or up is required to build PyTorch. Use the following two lines.
+set FORCE_PY27_BUILD=1
+REM As for CUDA 8, VS2015 Update 3 is also required to build PyTorch. Use the following two lines.
 set "PREBUILD_COMMAND=%VS140COMNTOOLS%\..\..\VC\vcvarsall.bat"
 set PREBUILD_COMMAND_ARGS=x64
 

setup.py

@@ -644,6 +644,10 @@ def run(self):
                           '/wd4305', '/wd4244', '/wd4190', '/wd4101', '/wd4996',
                           '/wd4275']
     if sys.version_info[0] == 2:
+        if not check_env_flag('FORCE_PY27_BUILD'):
+            print('The support for PyTorch with Python 2.7 on Windows is very experimental.')
+            print('Please set the flag `FORCE_PY27_BUILD` to 1 to continue build.')
+            sys.exit(1)
         # /bigobj increases number of sections in .obj file, which is needed to link
         # against libaries in Python 2.7 under Windows
         extra_compile_args.append('/bigobj')