RFC: Matrix Jobs syntax

At pytorch/pytorch we have a few extra things we’d need to make this more useful:

1. We have multiple matrices; for example, there is one matrix for Linux, and another for Windows. If you can only define a single top-level matrix that would be very troublesome
2. Our matrix jobs are actually multiple jobs with dependencies on one another. Can that be expressed in this syntax?
3. We sometimes want to “replicate” the matrix into multiple workflows, e.g., a workflow for PR builds and a workflow for nightlies (but with tweaks on parameters)

Here is an example, simplified CircleCI file that we’d be interested in using matrices on: https://github.com/pytorch/vision/blob/master/.circleci/config.yml#L294 and it would be interesting to see how this can be expressed in the new syntax.