import os
os.environ['CUDA_VISIBLE_DEVICES'] = ''

import numpy as np

from keras import layers
from keras.models import Model
from keras import backend as K

def get_model():
    inp = layers.Input(shape=(48,80,1))
    x = layers.Conv2D(32, 7, padding='same')(inp)
    x = layers.BatchNormalization()(x)
    x = layers.ReLU()(x)
    x = layers.Reshape([48, 80 * 32, 1])(x)
    x = layers.Lambda(lambda t: K.sum(t, axis=2))(x)
    return Model(inp, x)

if __name__ == '__main__':

    model = get_model()
    print(model.summary())

    model.compile(optimizer='adam', loss='mse')

    X = np.random.rand(32, 48, 80, 1)
    y = np.random.rand(32, 48, 1)

    model.fit(X, y, epochs=1, validation_data=(X, y))
    model.save('model.h5')

import cv2
net = cv2.dnn.readNetFromTensorflow('model.pb')
shape = (80,48)
image = cv2.imread('image.jpg', 0)
input = cv2.dnn.blobFromImage(image, 1, shape)
net.setInput(input)
output = net.forward()

Traceback (most recent call last):
  File "convert_utils/cv_pb_test.py", line 21, in <module>
    output = net.forward('lambda_1/Sum')
cv2.error: OpenCV(4.1.0) ../modules/dnn/src/dnn.cpp:524: error: (-2:Unspecified error) Can't create layer "lambda_1/Sum" of type "Sum" in function 'getLayerInstance'