Warning instead of error if nvcc is not found

docs/source/index.rst

@@ -19,7 +19,7 @@ Installation requires CUDA 9 or later, PyTorch 0.4 or later, and Python 3. Insta
    cd apex
    python setup.py install [--cuda_ext] [--cpp_ext]
 
-	       
+
 .. toctree::
    :maxdepth: 1
    :caption: AMP:  Automatic Mixed Precision

setup.py

@@ -32,25 +32,29 @@ if "--cpp_ext" in sys.argv:
 if "--cuda_ext" in sys.argv:
     from torch.utils.cpp_extension import CUDAExtension
     sys.argv.remove("--cuda_ext")
-    ext_modules.append(
-        CUDAExtension(name='fused_adam_cuda',
-                      sources=['apex/optimizers/csrc/fused_adam_cuda.cpp',
-                               'apex/optimizers/csrc/fused_adam_cuda_kernel.cu'],
-                      extra_compile_args={'cxx': ['-O3',],
-                                          'nvcc':['-O3', 
-                                                  '--use_fast_math']}))
-    ext_modules.append(
-        CUDAExtension(name='syncbn',
-                      sources=['csrc/syncbn.cpp',
-                               'csrc/welford.cu']))
-    ext_modules.append(
-        CUDAExtension(name='fused_layer_norm_cuda',
-                      sources=['apex/normalization/csrc/layer_norm_cuda.cpp',
-                               'apex/normalization/csrc/layer_norm_cuda_kernel.cu'],
-                      extra_compile_args={'cxx': ['-O3',],
-                                          'nvcc':['-maxrregcount=50',
-                                                  '-O3', 
-                                                  '--use_fast_math']}))
+
+    if torch.utils.cpp_extension.CUDA_HOME is None:
+        print("Warning:  nvcc is not available.  Ignoring --cuda-ext") 
+    else:
+        ext_modules.append(
+            CUDAExtension(name='fused_adam_cuda',
+                          sources=['apex/optimizers/csrc/fused_adam_cuda.cpp',
+                                   'apex/optimizers/csrc/fused_adam_cuda_kernel.cu'],
+                          extra_compile_args={'cxx': ['-O3',],
+                                              'nvcc':['-O3', 
+                                                      '--use_fast_math']}))
+        ext_modules.append(
+            CUDAExtension(name='syncbn',
+                          sources=['csrc/syncbn.cpp',
+                                   'csrc/welford.cu']))
+        ext_modules.append(
+            CUDAExtension(name='fused_layer_norm_cuda',
+                          sources=['apex/normalization/csrc/layer_norm_cuda.cpp',
+                                   'apex/normalization/csrc/layer_norm_cuda_kernel.cu'],
+                          extra_compile_args={'cxx': ['-O3',],
+                                              'nvcc':['-maxrregcount=50',
+                                                      '-O3', 
+                                                      '--use_fast_math']}))
 
 setup(
     name='apex',