Hi,
Indeed, in one case, you will create 256 graphs that work with one input.
In the second case, you will create only 4 graphs. but each of these 4 graphs is actually composed of 64 times the graph above and some Add operations at the end that sum the loss.
Indeed, in the second case you will use much more memory. Indeed, for the 64 iterations, you will create a single graph that just keep growing, and so you will use more and more memory.