Getting dynamic shape of tensors - tensorflow

Question

I want to create a custom metric in Keras with tensorflow backend.
Say, we have a minimalized metric, where we want to get the shape of a dynamic shaped tensor. I'm trying to do this as below:
def metric(y_true, y_pred):
y_num = (tf.shape(y_true))[0]
K.get_session().run(y_num)
return anything
But I always get the error message:
InvalidArgumentError: You must feed a value for placeholder tensor 'conv2d_47_target' with dtype float and shape [?,?,?,?]
[[Node: conv2d_47_target = Placeholder[dtype=DT_FLOAT, shape=[?,?,?,?], _device="/job:localhost/replica:0/task:0/device:GPU:0"]()]]
[[Node: metrics/metric/strided_slice/_1079 = _Recv[client_terminated=false, recv_device="/job:localhost/replica:0/task:0/device:CPU:0", send_device="/job:localhost/replica:0/task:0/device:GPU:0", send_device_incarnation=1, tensor_name="edge_11_metrics/metric/strided_slice", tensor_type=DT_INT32, _device="/job:localhost/replica:0/task:0/device:CPU:0"]()]]
I can't figure out, where could be the problem.
And if there is some problem in my code, how can I still get the shape some other way?
Any help would be highly appreciated.
UPDATE:
Model code:
ACTIVATION = "relu"
def convolution_block(x, filters, size, strides=(1,1), padding='same', activation=True):
x = Conv2D(filters, size, strides=strides, padding=padding)(x)
x = BatchNormalization()(x)
if activation == True:
x = Activation(ACTIVATION)(x)
return x
def residual_block(blockInput, num_filters=16):
x = Activation(ACTIVATION)(blockInput)
x = BatchNormalization()(x)
x = convolution_block(x, num_filters, (3,3) )
x = convolution_block(x, num_filters, (3,3), activation=False)
x = Add()([x, blockInput])
return x
# Build model
def build_model(input_layer, start_neurons, DropoutRatio = 0.5):
# 101 -> 50
conv1 = Conv2D(start_neurons * 1, (3, 3), activation=None, padding="same")(input_layer)
conv1 = residual_block(conv1,start_neurons * 1)
conv1 = residual_block(conv1,start_neurons * 1)
conv1 = Activation(ACTIVATION)(conv1)
pool1 = MaxPooling2D((2, 2))(conv1)
pool1 = Dropout(DropoutRatio/2)(pool1)
# 50 -> 25
conv2 = Conv2D(start_neurons * 2, (3, 3), activation=None, padding="same")(pool1)
conv2 = residual_block(conv2,start_neurons * 2)
conv2 = residual_block(conv2,start_neurons * 2)
conv2 = Activation(ACTIVATION)(conv2)
pool2 = MaxPooling2D((2, 2))(conv2)
pool2 = Dropout(DropoutRatio)(pool2)
# 25 -> 12
conv3 = Conv2D(start_neurons * 4, (3, 3), activation=None, padding="same")(pool2)
conv3 = residual_block(conv3,start_neurons * 4)
conv3 = residual_block(conv3,start_neurons * 4)
conv3 = Activation(ACTIVATION)(conv3)
pool3 = MaxPooling2D((2, 2))(conv3)
pool3 = Dropout(DropoutRatio)(pool3)
# 12 -> 6
conv4 = Conv2D(start_neurons * 8, (3, 3), activation=None, padding="same")(pool3)
conv4 = residual_block(conv4,start_neurons * 8)
conv4 = residual_block(conv4,start_neurons * 8)
conv4 = Activation(ACTIVATION)(conv4)
pool4 = MaxPooling2D((2, 2))(conv4)
pool4 = Dropout(DropoutRatio)(pool4)
# Middle
convm = Conv2D(start_neurons * 16, (3, 3), activation=None, padding="same")(pool4)
convm = residual_block(convm,start_neurons * 16)
convm = residual_block(convm,start_neurons * 16)
convm = Activation(ACTIVATION)(convm)
# 6 -> 12
deconv4 = Conv2DTranspose(start_neurons * 8, (3, 3), strides=(2, 2), padding="same")(convm)
uconv4 = concatenate([deconv4, conv4])
uconv4 = Dropout(DropoutRatio)(uconv4)
uconv4 = Conv2D(start_neurons * 8, (3, 3), activation=None, padding="same")(uconv4)
uconv4 = residual_block(uconv4,start_neurons * 8)
uconv4 = residual_block(uconv4,start_neurons * 8)
uconv4 = Activation(ACTIVATION)(uconv4)
# 12 -> 25
#deconv3 = Conv2DTranspose(start_neurons * 4, (3, 3), strides=(2, 2), padding="same")(uconv4)
deconv3 = Conv2DTranspose(start_neurons * 4, (3, 3), strides=(2, 2), padding="valid")(uconv4)
uconv3 = concatenate([deconv3, conv3])
uconv3 = Dropout(DropoutRatio)(uconv3)
uconv3 = Conv2D(start_neurons * 4, (3, 3), activation=None, padding="same")(uconv3)
uconv3 = residual_block(uconv3,start_neurons * 4)
uconv3 = residual_block(uconv3,start_neurons * 4)
uconv3 = Activation(ACTIVATION)(uconv3)
# 25 -> 50
deconv2 = Conv2DTranspose(start_neurons * 2, (3, 3), strides=(2, 2), padding="same")(uconv3)
uconv2 = concatenate([deconv2, conv2])
uconv2 = Dropout(DropoutRatio)(uconv2)
uconv2 = Conv2D(start_neurons * 2, (3, 3), activation=None, padding="same")(uconv2)
uconv2 = residual_block(uconv2,start_neurons * 2)
uconv2 = residual_block(uconv2,start_neurons * 2)
uconv2 = Activation(ACTIVATION)(uconv2)
# 50 -> 101
#deconv1 = Conv2DTranspose(start_neurons * 1, (3, 3), strides=(2, 2), padding="same")(uconv2)
deconv1 = Conv2DTranspose(start_neurons * 1, (3, 3), strides=(2, 2), padding="valid")(uconv2)
uconv1 = concatenate([deconv1, conv1])
uconv1 = Dropout(DropoutRatio)(uconv1)
uconv1 = Conv2D(start_neurons * 1, (3, 3), activation=None, padding="same")(uconv1)
uconv1 = residual_block(uconv1,start_neurons * 1)
uconv1 = residual_block(uconv1,start_neurons * 1)
uconv1 = Activation(ACTIVATION)(uconv1)
uconv1 = Dropout(DropoutRatio/2)(uconv1)
output_layer = Conv2D(1, (1,1), padding="same", activation="sigmoid")(uconv1)
#output_layer = Conv2D(1, (1,1), padding="same", activation="sigmoid")(uconv1)
return output_layer