Unskip Integ test `test_hp_contract_mpi_script` for SageMaker Train

Integ test `test_hp_contract_mpi_script` in [test_model_trainer.py](https://github.com/aws/sagemaker-python-sdk/blob/master/sagemaker-train/tests/integ/train/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.

Start in sagemaker-train/tests/integ/train/test_model_trainer.py and locate the skipped test_hp_contract_mpi_script test. Run the integration test to identify its configuration or runtime failures, then address the reported issues and remove the skip. Done means the test passes successfully for SageMaker Train.

Written by the indexing model from the issue text.