Course 4 - Week 2 - Assignment 2 - Ex3

When we fine-tune our model, what does model2.layer[4] do? Why are we calling out the 4th layer in model2 and assigning it to variable base_model?

model2.layers[4] is referring to the mobilenet model.
We want to fine-tune layers of this model starting at index 120.

Here’s the configuration of model2:

Model: "model"
_________________________________________________________________
 Layer (type)                Output Shape              Param #   
=================================================================
 input_5 (InputLayer)        [(None, 160, 160, 3)]     0         
                                                                 
 sequential_3 (Sequential)   (None, 160, 160, 3)       0         
                                                                 
 tf.math.truediv (TFOpLambda  (None, 160, 160, 3)      0         
 )                                                               
                                                                 
 tf.math.subtract (TFOpLambd  (None, 160, 160, 3)      0         
 a)                                                              
                                                                 
 mobilenetv2_1.00_160 (Funct  (None, 5, 5, 1280)       2257984   
 ional)                                                          
                                                                 
 global_average_pooling2d_1   (None, 1280)             0         
 (GlobalAveragePooling2D)                                        
                                                                 
 dropout (Dropout)           (None, 1280)              0         
                                                                 
 dense (Dense)               (None, 1)                 1281      
                                                                 
=================================================================
Total params: 2,259,265
Trainable params: 1,281
Non-trainable params: 2,257,984
_________________________________________________________________