Fix build error for CoreML Split op (#19099)
### Description
<!-- Describe your changes. -->
The `split` input of the Split op is int64_t. Fixing that resolves a
type mismatch build error on Windows when CoreML is enabled (for
debugging the partitioning code).
### 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. -->
Fix build error
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Co-authored-by: Edward Chen <18449977+edgchen1@users.noreply.github.com>
