Your model expects a 4 dimensional input, i.e. [batch_size, channels, height, width].
Since you are loading a single image, the batch dimension is missing.
You can add it with img = img.unsqueeze(0).
Also, I would recommend to transform your numpy array to a tensor using torch.from_numpy.
Note that you are not normalizing the image either, which will yield bad results.
In the code you’re using, the data is transformed using:
[transforms.ToTensor(), 
transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5))])

I would suggest to do the same in your test phase.