Extracting nested layer features (from a pretrained model) with keras sequential API

Question

I have the following simple model for transfer learning with a pretrained model (VGG16) without the FC layers, followed by a few new layers, defined with keras sequential API.

IMG_SHAPE = (224, 224, 3)

# vgg16
pretrained_model = tf.keras.applications.vgg16.VGG16(
        weights='imagenet', 
        include_top=False, 
        input_shape=IMG_SHAPE,
    )
# freeze pretrained layers
pretrained_model.trainable = False

model = tf.keras.Sequential([
    pretrained_model,
    tf.keras.layers.BatchNormalization(),
    tf.keras.layers.Flatten(),
    tf.keras.layers.Dense(3, activation='softmax'),
])

Notice the summary of the model does not show the internal layers for VGG16 :

model.summary()

#Model: "sequential"
#_________________________________________________________________
# Layer (type)                Output Shape              Param #   
#=================================================================
# vgg16 (Functional)          (None, 4, 4, 512)         14714688  
#                                                                 
# batch_normalization (BatchN  (None, 4, 4, 512)        2048      
# ormalization)                                                   
#                                                                 
# flatten (Flatten)           (None, 8192)              0         
                                                                 
# dense (Dense)               (None, 2)                 16386     
                                                                 
#=================================================================
#Total params: 14,733,122
#Trainable params: 17,410
#Non-trainable params: 14,715,712

I have trained the above model on my custom dataset and got the desired accuracy on my test dataset with transfer learning.

Now, let's say I want to create a new model (eg, to compute the activation map) that accepts the inputs as the input to the previous model, and as outputs, I want an intermediate output (by extracting the features at a convolution layer, eg, block5_conv3 , of the pretrained model) along with the output from the previous model. That's something where I am getting stuck and I am getting errors. For example, I have defined the new model like the following:

grad_model = tf.keras.models.Model(
        [pretrained_model.inputs], 
        [pretrained_model.get_layer('block5_conv3').output, model.output]
    )

where I am getting the following error:

ValueError: Graph disconnected: cannot obtain value for tensor KerasTensor(type_spec=TensorSpec(shape=(None, 150, 150, 3), dtype=tf.float32, name='vgg16_input'), name='vgg16_input', description="created by layer 'vgg16_input'") at layer "vgg16". The following previous layers were accessed without issue: ['block1_conv1', 'block1_conv2', 'block1_pool', 'block2_conv1', 'block2_conv2', 'block2_pool', 'block3_conv1', 'block3_conv2', 'block3_conv3', 'block3_pool', 'block4_conv1', 'block4_conv2', 'block4_conv3']

or like the following:

grad_model = tf.keras.models.Model(
        [model.inputs], 
        [pretrained_model.get_layer('block5_conv3').output, model.output]
    )

where I am getting the following error:

ValueError: Graph disconnected: cannot obtain value for tensor KerasTensor(type_spec=TensorSpec(shape=(None, 150, 150, 3), dtype=tf.float32, name='input_1'), name='input_1', description="created by layer 'vgg16'") at layer "block1_conv1". The following previous layers were accessed without issue: []

I have also tried to set the names of the input layer of the model and the pretrained model nested inside so that the input layer names are the same:

pretrained_model.layers[0]._name = model.layers[0]._name

but getting the same error.

I think the model structure can be changed (eg, using keras functional API, etc.), to define the grad_model , but not sure how to. Also, I am more interested to know if there is a way to resolve the issues without changing the model structure / without requiring me to retrain.

Answer 1

Following @M.Innat's comment so far I could solve the issue with keras functional API (note that the number of parameters remains unchanged) and re-training:

inputs = tf.keras.Input(shape=IMG_SHAPE)

x = keras.applications.xception.Xception(
    input_tensor=inputs, 
    include_top=False,
    weights='imagenet'
)

x.trainable = False

x = tf.keras.layers.BatchNormalization()(x.output) 
x = tf.keras.layers.Flatten()(x)
x = tf.keras.layers.Dense(2, activation='softmax')(x)

model = tf.keras.Model(inputs, x)

model.summary()

# Model: "model_1262"
#_________________________________________________________________
# Layer (type)                Output Shape              Param #   
#=================================================================
# input_3 (InputLayer)        [(None, 150, 150, 3)]     0        
#                                                                 
# block1_conv1 (Conv2D)       (None, 150, 150, 64)      1792      
#                                                                 
# block1_conv2 (Conv2D)       (None, 150, 150, 64)      36928     
#                                                                 
# block1_pool (MaxPooling2D)  (None, 75, 75, 64)        0         
#                                                                 
# block2_conv1 (Conv2D)       (None, 75, 75, 128)       73856     
#                                                                 
# block2_conv2 (Conv2D)       (None, 75, 75, 128)       147584    
#                                                                 
# block2_pool (MaxPooling2D)  (None, 37, 37, 128)       0         
#                                                                 
# block3_conv1 (Conv2D)       (None, 37, 37, 256)       295168    
#                                                                 
# block3_conv2 (Conv2D)       (None, 37, 37, 256)       590080    
#                                                                 
# block3_conv3 (Conv2D)       (None, 37, 37, 256)       590080    
#                                                                 
# block3_pool (MaxPooling2D)  (None, 18, 18, 256)       0         
#                                                                 
# block4_conv1 (Conv2D)       (None, 18, 18, 512)       1180160   
#                                                                 
# block4_conv2 (Conv2D)       (None, 18, 18, 512)       2359808   
#                                                                 
# block4_conv3 (Conv2D)       (None, 18, 18, 512)       2359808   
#                                                                 
# block4_pool (MaxPooling2D)  (None, 9, 9, 512)         0         
#                                                                 
# block5_conv1 (Conv2D)       (None, 9, 9, 512)         2359808   
#                                                                 
# block5_conv2 (Conv2D)       (None, 9, 9, 512)         2359808   
#                                                                 
# block5_conv3 (Conv2D)       (None, 9, 9, 512)         2359808   
#                                                                 
# block5_pool (MaxPooling2D)  (None, 4, 4, 512)         0         
#                                                                 
# batch_normalization_9 (Batc  (None, 4, 4, 512)        2048      
# hNormalization)                                                 
#                                                                 
# flatten_1 (Flatten)         (None, 8192)              0         
#                                                                 
# dense_1 (Dense)             (None, 2)                 16386     
#                                                                 
#=================================================================
#Total params: 14,733,122
#Trainable params: 17,410
#Non-trainable params: 14,715,712
#_________________________________________________________________

and the following code to extract the intermediate features for class activation map worked:

grad_model = tf.keras.models.Model(
        [model.inputs], [model.get_layer('block5_conv3').output, model.output] 
    )