remove MULTI_GPU (#25509)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25509
Trying to reduce the number of build parameters to simplify the config.
This one is purely derived from the build environment, so we can have
the CI scripts just compute it.
Test Plan: Imported from OSS
Differential Revision: D17143343
Pulled By: suo
fbshipit-source-id: 7837607b7b18a9233fd8657dc9c63539c0194110
