Clean up the ci/ directory. This means getting rid of the travis/ path completely and moving the files into sensible subdirectories.
Details:
- Moves everything under ci/travis into subdirectories, e.g. ci/build, ci/lint, etc.
- Minor adjustments to some scripts (variable renames)
- Removes the outdated (unused) asan tests
This updates the GPU image to run on the same Ubuntu version as the regular (non-GPU) image. This implicitly updates cmake etc for compatibility with newer versions of downstream libraries, e.g. Horovod.
Instead of installing dependencies in each Buildkite job, let's move this to the Dockerfile instead.
This will update GPU tests to always use Python 3.7.
* Revert "Revert "[Build] include minimal debug info in C++ build; upgrade clang-format to 12 (#18840)" (#18886)"
This reverts commit f851a072f3.
* use gcc 8
