[1.8] Do not print warning if CUDA driver not found (pytorch#51806) (pytorch#52050)

Summary:
It frequently happens when PyTorch compiled with CUDA support is installed on machine that does not have NVIDIA GPUs.

Fixes pytorch#47038

Pull Request resolved: pytorch#51806

Reviewed By: ezyang

Differential Revision: D26285827

Pulled By: malfet

fbshipit-source-id: 9fd5e690d0135a2b219c1afa803fb69de9729f5e

Author: malfet

c10/cuda/CUDAFunctions.cpp

@@ -16,7 +16,7 @@ int32_t driver_version() {
   return driver_version;
 }
 
-int device_count_impl() {
+int device_count_impl(bool fail_if_no_driver) {
   int count;
   auto err = cudaGetDeviceCount(&count);
   if (err == cudaSuccess) {
@@ -34,6 +34,11 @@ int device_count_impl() {
     case cudaErrorInsufficientDriver: {
       auto version = driver_version();
       if (version <= 0) {
+        if (!fail_if_no_driver) {
+          // No CUDA driver means no devices
+          count = 0;
+          break;
+        }
         TORCH_CHECK(
             false,
             "Found no NVIDIA driver on your system. Please check that you "
@@ -95,9 +100,9 @@ DeviceIndex device_count() noexcept {
   // initialize number of devices only once
   static int count = []() {
     try {
-      auto result = device_count_impl();
+      auto result = device_count_impl(/*fail_if_no_driver=*/false);
       TORCH_INTERNAL_ASSERT(result <= std::numeric_limits<DeviceIndex>::max(), "Too many CUDA devices, DeviceIndex overflowed");
-      return device_count_impl();
+      return result;
     } catch (const c10::Error& ex) {
       // We don't want to fail, but still log the warning
       // msg() returns the message without the stack trace
@@ -110,7 +115,7 @@ DeviceIndex device_count() noexcept {
 
 DeviceIndex device_count_ensure_non_zero() {
   // Call the implementation every time to throw the exception
-  int count = device_count_impl();
+  int count = device_count_impl(/*fail_if_no_driver=*/true);
   // Zero gpus doesn't produce a warning in `device_count` but we fail here
   TORCH_CHECK(count, "No CUDA GPUs are available");
   return static_cast<DeviceIndex>(count);