Group build args (#24337)
1. Split build.py to two files, because currently the file is over 3000
lines. This PR moves 900 of them to a new file.
2. Put the build args into groups. It makes more explicit that "--x86",
"--arm", "--arm64" and "--arm64ec" args are for Windows only.
3. Remove the "--use_avx512" and "--gen-api-doc" build args, as they are
not referenced anywhere. "--gen-api-doc" was for generating documents
for pytorch frontend.
4. Remove MPI related build flags.
5. Delete tools/ci_build/github/pai/orttraining-ci.yml
6. Remove --use_preinstalled_eigen and --eigen_path. Now we have a more
unified approach for all ORT's dependencies (not just eigen). See
https://onnxruntime.ai/docs/build/dependencies.html for more
information.
7. Windows specific build options won't show up on non-Windows
platforms. The same for macOS.
