improve libtorch install docs with GPU note (pytorch#17299)

Summary:
Fixes pytorch#15702
Pull Request resolved: pytorch#17299

Differential Revision: D14149712

Pulled By: soumith

fbshipit-source-id: 5b83110bb00e4d4dad04c1f293c2b52e41711f11

docs/cpp/source/installing.rst

@@ -19,6 +19,8 @@ example:
   wget https://download.pytorch.org/libtorch/nightly/cpu/libtorch-shared-with-deps-latest.zip
   unzip libtorch-shared-with-deps-latest.zip
 
+Note that the above link has CPU-only libtorch. If you would like to download a GPU-enabled 
+libtorch, find the right link in the link selector on https://pytorch.org
 
 Next, we can write a minimal CMake build configuration to develop a small
 application that depends on LibTorch. CMake is not a hard requirement for using