Add GitHub Workflow CI

[EnricoMi]

A GitHub Workflow for running the CPU test pipeline.

CPU tests finish in 1:30 on GitHub, GPU tests finish in 1:15 on Buildkite.

The workflow file is generated by Python script .github/gen-workflow-ci.py based on the output of .buildkite/gen-pipeline.sh, so that we implement test logic only in one place.

These are the implemented features:

• builds all CPU images and runs appropriate tests
• builds GPU images but does not run any tests
• tests for GPU images are run on Buildkite after all above builds and tests succeed
• tests three images on macOS (replaces TravisCI)
• images where all frameworks are nightly versions do not prevent the Buildkite tests
• PRs without code changes (e.g. docs only) to not run the full build and test cycle (that is what Buildkite pipeline does)
• test results are downloaded from Buildkite and together with GitHub test results posted to the pull request
• build and test steps have timeouts and are retried three times
• workflow detects when its yaml file is not up-to-date
• earlier workflow runs on the same branch are cancelled
• triggering the Buildkite builds and publishing test results for fork PRs (can only be tested after merging to master)
• daily master build schedule
• Docker images are build on PR (not pushed), as well as master and version tags (and pushed to Dockerhub), but only if the build and test jobs succeed
• Docs and example sync workflows integrated into the CI workflow

Limitations:

• Buildkite build always has author "Travis Addair" and build title "GPU Tests triggered by GitHub": we may use a service account here.
• ONECCL image does not build and is excluded (af2f3bb): https://github.com/horovod/horovod/runs/2221724507?check_suite_focus=true (Make Horovod build with OneCCL in GitHub CI #2846)
• GitHub only provides 20 consecutive jobs, so concurrent workflows may block new runs. There can be two concurrent CI workflows, which allows two more jobs.

Missing:

• disable Buildkite triggered by git pushs (with this merged into master the buildkite pipeline is empty)