build: Have ability to split the build in CI

[mc-nv]

MOTIVATION:
Triton Server CI is monolithic, I would like to add ability splitting the CI and build it per backend if needed

CHANGES:

1. Added flags --ci-split and --cache-from-map
2. Update logic of file creation in case of flag --ci-split

VERIFICATION:

$ ./build.py --enable-all --split-build --dryrun
$ ls build
cmake_build                 cmake_build_backend_ensemble  cmake_build_backend_onnxruntime  cmake_build_backend_pytorch  cmake_build_backend_tensorflow  cmake_build_cache_redis  Dockerfile
cmake_build_agent_checksum  cmake_build_backend_fil       cmake_build_backend_openvino     cmake_build_backend_repeat   cmake_build_backend_tensorrt    cmake_build_collect      Dockerfile.buildbase
cmake_build_backend_dali    cmake_build_backend_identity  cmake_build_backend_python       cmake_build_backend_square   cmake_build_cache_local         docker_build             Dockerfile.cibase

$ ./build.py --enable-all --dryrun
$ ls build
cmake_build  docker_build  Dockerfile  Dockerfile.buildbase  Dockerfile.cibase

[nv-kmcgill53]

Just an API comment, since API's are sticky you try to get them right the first time because the overhead of changing them later is high.

[nv-kmcgill53]

I understand that it was called a map before, but it's really just a list. What do you think about changing this to something like --cache-image or --cache-images? We can make use of the multiple arguments in a couple of different ways. We can use the action="append" or the nargs='+' fields (SO comment for example). I might lean towards the --cache-image with action="append" API just because we have precedent.

[mc-nv]

I think name was used to keep consistency with original command which is relates to: docker build --cache-from

$ docker build --help | grep cache-from
      --cache-from stringArray        External cache sources (e.g., "user/app:cache", "type=local,src=path/to/dir")

[nv-kmcgill53]

How about --cache-from-list?