update Anaconda download link (#19794)
Summary:
Now `https://www.continuum.io/` is redirected to `https://www.anaconda.com` and old Anaconda download link `https://www.continuum.io/downloads` is dead. This PR update it to `https://www.anaconda.com/distribution/#download-section`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19794
Differential Revision: D15099538
Pulled By: soumith
fbshipit-source-id: 967dcda34d9d446c0d26c0014f10cc710f69a0c5
