Update Circle CI docker image to pytorch 1.6

[vfdev-5]

🚀 Feature

Following the release of 1.6 version of PyTorch, let's update Circle CI docker image :

• https://github.com/pytorch/ignite/blob/master/.circleci/config.yml
  and add 1.5.1 version to pytorch compatibilty versions checking :
• https://github.com/pytorch/ignite/blob/master/.github/workflows/pytorch-version-tests.yml