Use CUDA 11.2 for nightly docker build. (#51990)
Summary:
Set CUDA_VERSION to 11.2.0 since Nvidia name their docker image on Ubuntu 18.04 to be nvidia/cuda:11.2.0-cudnn8-devel-ubuntu18.04.
Note that cudatoolkit 11.2.0 is not yet on [conda](https://repo.anaconda.com/pkgs/main/linux-64/), and we need to wait for that before merging this PR.
- https://hub.docker.com/r/nvidia/cuda/
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51990
Reviewed By: samestep
Differential Revision: D26371193
Pulled By: xuzhao9
fbshipit-source-id: 76915490dc30ddb03ceeeadb3c45a6c02b60401e
