Unskip Integ test `test_hp_contract_mpi_script` for SageMaker Train

[lucasjia-aws]

Integ test test_hp_contract_mpi_script in test_model_trainer.py was skipped for some reson, we need to unskip it since it is critical test. While there are some bugs/config issues in the test, we need to fix it until it get passed.