Fix ops registration on windows (#3380)

* On MSVC, instruct the linker not to drop the _register_ops symbol

* Remove workaround for ops registration on windows
Co-authored-by: Francisco Massa <fvsmassa@gmail.com>

test/tracing/frcnn/test_frcnn_tracing.cpp

@@ -3,11 +3,6 @@
 #include <torchvision/vision.h>
 #include <torchvision/ops/nms.h>
 
-#ifdef _WIN32
-// Windows only
-// This is necessary until operators are automatically registered on include
-static auto _nms = &vision::ops::nms;
-#endif
 
 int main() {
   torch::DeviceType device_type;

torchvision/csrc/macros.h

@@ -15,6 +15,7 @@
 #else
 #ifdef _MSC_VER
 #define VISION_INLINE_VARIABLE __declspec(selectany)
+#define HINT_MSVC_LINKER_INCLUDE_SYMBOL
 #else
 #define VISION_INLINE_VARIABLE __attribute__((weak))
 #endif

torchvision/csrc/vision.h

@@ -7,9 +7,10 @@ namespace vision {
 VISION_API int64_t cuda_version();
 
 namespace detail {
-// Dummy variable to reference a symbol from vision.cpp.
-// This ensures that the torchvision library and the ops registration
-// initializers are not pruned.
-VISION_INLINE_VARIABLE int64_t _cuda_version = cuda_version();
+extern "C" VISION_INLINE_VARIABLE auto _register_ops = &cuda_version;
+#ifdef HINT_MSVC_LINKER_INCLUDE_SYMBOL
+#pragma comment(linker, "/include:_register_ops")
+#endif
+
 } // namespace detail
 } // namespace vision