[XNNPACK] Allow VCVT Operators (#97385)
Allow VCVT operators to allow operators that change datatype.
We want xnn_define_convert, to convert from one data type to another. Explanation for usage provided in follow up diff
Differential Revision: [D43844094](https://our.internmc.facebook.com/intern/diff/D43844094/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D43844094/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97385
Approved by: https://github.com/salilsdesai
