Include Keras tensor in the allowed types (#14155)
* Include KerasTensor in allowed types
- This allows propagating symbolic tensors through TFBert models and layers' call(),
which allows converting the subclass models to functional models.
* Style pass
Co-authored-by: Sergio Valcarcel Macua <sergiov@graphcore.ai>
Co-authored-by: matt <rocketknight1@gmail.com>
