How can i group and sum elements in matrix, without using itertools.groupby?

Question

the problem that I am facing is simple. I have matrix as list of lists like this:

[[0, 0, 1, 0, 0, 0, 0], [0, 1, 0, 1, 0, 0, 1], [1, 1, 1, 1, 1, 1, 1], [0, 0,