### 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>
### Description
Implement Objective-C binding for `ORTCheckPoint`. Additionally,
- Modify `onnxruntime_objectivec.cmake` to only include training header
and sources when training flag is enabled
- Enable objective-c binding for `orttraining-mac-ci-pipeline`
### Motivation and Context
This PR is part of implementing Objective-C bindings for training API.
It implements objective-c binding for ORTCheckPoint class. The
objective-C API closely resembles the C++ API.
**Note**: The test for saving checkpoint is skipped as it requires use
of training session. It will be added when the objective-c binding for
`ORTTrainingSession` is added.
