Fix gradient builder for Cast (#11008)
* fix grad builder for cast
* reviw comments
Co-authored-by: Aishwarya Bhandare <aibhanda@microsoft.com@orttrainingdev8.d32nl1ml4oruzj4qz3bqlggovf.px.internal.cloudapp.net>
Co-authored-by: Aishwarya Bhandare <aibhanda@microsoft.com@orttrainingdev7.d32nl1ml4oruzj4qz3bqlggovf.px.internal.cloudapp.net>
