Can't figure out layer shape

Course Q&A TensorFlow: Advanced Techniques Specialization Advanced Computer Vision with TensorFlow
2

I am not noticing anywhere that an Input is defined. You need something like

    #define an input layer 
input_layer = keras.Input(shape=(784,), name="my_input_layer")

    #inform the model about its inputs
sequential_model_def(input_layer)

There’s a little more about this here: Tips for troubles with Sequential and Functional API syntax