[seq2seq Examples] Use _step instead of generate for val, test
Addresses a workaround I proposed in #6589 for limiting the memory consumption in `.generate` steps.

@sshleifer 
@sshleifer : Does it make sense to still add this? If yes, I can rebase and update based on Suraj's comments earlier. If not, will close.<|||||>Let's hold off for now. I think passing --eval_num_beams=1 is close enough to equivalent.
Thanks for trying!