Creates a LinearModel (inherits from keras.training.Model) that creates a linear

Creates a LinearModel (inherits from keras.training.Model) that creates a linear
model.

Had to modify the __call__ method in the base layer class so that it could work with feature style inputs in which case we lazily convert the inputs to tensors instead of providing tensors as inputs upfront.

PiperOrigin-RevId: 191655445