Upgrade pytorch nightly docker python version to 3.8 (#80051)
We are planning to use nightly docker as a base to build torchbench nightly docker. Torchbench is using Python 3.8, so we would like PyTorch nightly docker to use 3.8 by default as well.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80051
Approved by: https://github.com/atalman
