1. Fix Nuget package build break caused by #6225
2. Delete Dockerfile.centos_gpu. It is not used anywhere.
3. Fix Linux CUDA 10.2 build error caused by glibc upgrade
Update training Python packaging build to use get_docker_image.py.
Remove BUILD_EXTR_PAR docker build argument.
Update get_docker_image.py to check again for the image in the cache after building and before pushing to reduce the chance of a redundant push.
