### Description
- Implement Objective-C binding for `ORTTrainingSession`
- Add `ORTUtils` utility class to handle conversion between C++ and
Objective-C types
- Add test case for saving checkpoint
- Add unit test cases for `ORTTrainingSession`
### Motivation and Context
This PR is part of implementing Objective-C bindings for training API.
It implements objective-c binding for training session. The objective-C
API closely resembles the C++ API.
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Co-authored-by: Edward Chen <18449977+edgchen1@users.noreply.github.com>
