Hi,
The output of your last Conv2d  would be like (N, 64, 7, 7), where N stands for batch_size, 64 for number of channels and 7x7, the height and width of the image.
So, now Flatten() will convert this into shape (N, 64 x 7 x 7). Now, when it will go to the first Linear, the output will be (N, 100) and after second Linear (N, 10).
Please do ask if you are still confused…