How to enumerate combinations in parallel
Since you just need to select $k$ rows, this becomes the same as choosing $k$ elements of a given set of $n$ elements, say $\{1,2,\dots,n\}$.

You can generate $n \choose k$ combinations in lexicographic order (i.e. starting with $\{1,2,3, \dots, k\}$ and ending with $\{n-k+1, ..., n-1,n\}$), with the next one depending only on the previous one.

The fact that the order is predictable, and that you only need the previous state, allows you to split the output space, and apply independent parallel computations to each.

There are plenty of algorithms available on the web for lexicographic generation, for instance Frank Ruskey's book: Combinatorial Generation has a chapter (4) on lexicographic algorithms, and for your purposes, you can use Algorithm 4.4 which appears on page 58.