Revert "Add std::variant backport as c10::variant (#26836)" (#27277)
Summary:
This reverts commit 0cd188035a27fc38ce1e8eee205f6d47cd7650e6.
As reported by jerryzh168 and pritamdamania87, mpark::variant doesn’t compile with gcc 7.3.1 on fb devserver and throws error similar to https://github.com/mpark/variant/issues/43. (However, it doesn’t fail with gcc 7.3.1 in OSS CI, based on https://circleci.com/api/v1.1/project/github/pytorch/pytorch/2995606/output/107/0?file=true)
A plausible workaround is to upgrade devserver to devtoolset-8, but that would in turn causes CUDA build to complain:
```
/usr/local/cuda/bin/../targets/x86_64-linux/include/crt/host_config.h:119:2: error: #error -- unsupported GNU version! gcc versions later than 7 are not supported!
#error -- unsupported GNU version! gcc versions later than 7 are not supported!
```
(Thanks pritamdamania87 for the report!)
The solution for now is to revert the mpark::variant addition, and I will find alternatives that will work with gcc 7.3.1 on fb devserver.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27277
Differential Revision: D17739804
fbshipit-source-id: ad945b3d86ab7ddbff58f4ecab95e0e1ac725ae9
