Robust determination of cudnn library and relevant conda packages. (#16859)
Summary:
This PR implements:
1. a fix to issue #12174 - determine the location of cudnn library using `ldconfig`
2. a fix to determine the installed conda packages (in recent versions of conda, the command `conda` is a Bash function that cannot be called within a python script, so using CONDA_EXE environment variable instead)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16859
Differential Revision: D14000399
Pulled By: soumith
fbshipit-source-id: 905658ecacb0ca0587a162fade436de9582d32ab
