Default the GPU backend to DEX. (#4327)

CMakeLists.txt

@@ -620,7 +620,7 @@ if (NGRAPH_MLIR_ENABLE)
     include(cmake/external_mlir.cmake)
 endif()
 
-if ((NGRAPH_GPU_ENABLE OR NGRAPH_CPU_ENABLE) AND NOT NGRAPH_DEX_ONLY)
+if (NGRAPH_CPU_ENABLE AND NOT NGRAPH_DEX_ONLY)
     set(NGRAPH_CODEGEN_ENABLE TRUE)
 else()
     set(NGRAPH_CODEGEN_ENABLE FALSE)

src/ngraph/runtime/gpu/CMakeLists.txt

@@ -51,10 +51,6 @@ set(SRC
     op/rnn.cpp
     )
 
-if (NOT NGRAPH_DEX_ONLY)
-    list(APPEND SRC gpu_external_function.cpp)
-endif()
-
 set(CUDA_INC
     ${PROJECT_SOURCE_DIR}/src/
     )
@@ -152,11 +148,6 @@ if (NGRAPH_GPU_ENABLE)
             SOVERSION ${NGRAPH_API_VERSION})
     endif()
     target_link_libraries(gpu_backend PUBLIC ngraph)
-    if (NGRAPH_DEX_ONLY)
-        target_compile_definitions(gpu_backend PRIVATE "NGRAPH_DEX_ONLY")
-    else()
-        target_link_libraries(gpu_backend PUBLIC codegen)
-    endif()
     find_library(CUDA_nvrtc_LIBRARY nvrtc
         PATH_SUFFIXES lib lib64 cuda/lib cuda/lib64 lib/x64)
     find_library(CUDA_cuda_LIBRARY cuda

src/ngraph/runtime/gpu/gpu_compiled_function.cpp

@@ -120,31 +120,7 @@ std::shared_ptr<runtime::gpu::GPUCompiledFunction> runtime::gpu::GPUCompiledFunc
     const std::shared_ptr<ngraph::Function>& function,
     const std::shared_ptr<GPU_Backend::BackendContext>& shared_context)
 {
-#if defined(NGRAPH_DEX_ONLY)
     return std::make_shared<runtime::gpu::GPUInternalFunction>(function, shared_context);
-#else
-    // For now codegen is default unless explicitly disabled
-    bool use_codegen = true;
-    if (auto env = std::getenv("NGRAPH_CODEGEN"))
-    {
-        std::string env_codegen(env);
-        for (auto& opt : get_case_variants({"0", "false"}))
-        {
-            if (env_codegen == opt)
-            {
-                use_codegen = false;
-            }
-        }
-    }
-    if (use_codegen)
-    {
-        return std::make_shared<runtime::gpu::GPUExternalFunction>(function, shared_context);
-    }
-    else
-    {
-        return std::make_shared<runtime::gpu::GPUInternalFunction>(function, shared_context);
-    }
-#endif
 }
 
 void runtime::gpu::GPUCompiledFunction::compile()