Fix Training CPU docker image name to avoid unnecessary rebuilding (microsoft#19973)

### Description
The docker image name was fixed, but the docker argument was different
in different job.
It would trigger rebuilding the docker image almost every time!!!

Author: mszhanyi

tools/ci_build/github/azure-pipelines/orttraining-py-packaging-pipeline-cpu.yml

@@ -46,7 +46,7 @@ stages:
             --build-arg PYTHON_VERSION=$(PythonVersion)
             --build-arg INSTALL_DEPS_EXTRA_ARGS=-tu
             --build-arg BUILD_UID=$(id -u)
-          Repository: onnxruntimetrainingcpubuild
+          Repository: onnxruntimetrainingcpubuild_$(PythonVersion)
 
       - task: CmdLine@2
         displayName: 'build onnxruntime'