Allow forcing GPU build with FORCE_CUDA=1 (#927)

* Allow forcing GPU build with FORCE_CUDA=1

This is convenient to e.g. build with GPU support inside a docker image

* Document FORCE_CUDA

README.rst

@@ -41,6 +41,9 @@ From source:
     # or, for OSX
     # MACOSX_DEPLOYMENT_TARGET=10.9 CC=clang CXX=clang++ python setup.py install
 
+By default, GPU support is built if CUDA is found and ``torch.cuda.is_available()`` is true.
+It's possible to force building GPU support by setting ``FORCE_CUDA=1`` environment variable,
+which is useful when building a docker image.
 
 Image Backend
 =============

setup.py

@@ -92,7 +92,7 @@ def get_extensions():
     define_macros = []
 
     extra_compile_args = {}
-    if torch.cuda.is_available() and CUDA_HOME is not None:
+    if (torch.cuda.is_available() and CUDA_HOME is not None) or os.getenv('FORCE_CUDA', '0') == '1':
         extension = CUDAExtension
         sources += source_cuda
         define_macros += [('WITH_CUDA', None)]