coreml: Fix Gather fp16 support (#26442)
### Description
<!-- Describe your changes. -->
The coreml gather MLProgram operator supports fp16, but the check was
missing in `GatherOpBuilder::HasSupportedInputsImpl()`
### 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. -->
We use `Gather` in LeelaChessZero.
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Co-authored-by: borg323 <borg323@users.noreply.github.com>
