It seems the input to model should have 3 channels but the input is having 8 channels.
if you look at your input size the size is torch.Size([8, 600, 800]).It means you are having 8 channels but you are saying batch size is 8.so if your input is black and white so firstly try to apply unsqueeze(1) to that image_tensor  so your shape becomes [8,1,600,800].Then reply me what image are you using like black and white or color etc