Fix error in mixed precision training of `TFCvtModel` (#22267)
* Make sure CVT can be trained using mixed precision
* Add test for keras-fit with mixed-precision
* Update tests/models/cvt/test_modeling_tf_cvt.py
Co-authored-by: Matt <Rocketknight1@users.noreply.github.com>
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Co-authored-by: gcuder <Gerald.Cuder@iacapps.com>
Co-authored-by: Matt <Rocketknight1@users.noreply.github.com>