Update gradient ops tests (#18783)
### Description
<!-- Describe your changes. -->
TrainingSession has been deprecated for a while now, but the gradient
ops tests are still using training session. This PR updates these tests
to use inference session instead of training session.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
This will enable us to remove all the training session related
deprecated code from the repo.
