How I can do round robin distribution? I understand that I can write something like:

every 20 ticks do
    INPUT stone FROM in
    OUTPUT 10 TO EACH out
end

But if "in" is receiving less than 10*out items per second then all items goes into the first "out" only.

added in 4.14.0, see example programs for usage

not yet implemented, but if you want to avoid stuffing one inventory when others are empty try

every 20 ticks do
  input stone from in
  output retain 1 to each out
end

which should ensure each inventory keeps at most 1 stone