Fix EOS ngrams

[sebpuetz]

The ngram vocab currently brackets the <\s> marker and extracts ngrams from "<<\s>>". Those subwords aren't trained because the indices are never added to the subwords Vec in the vocab via:

            if word.word() == util::EOS {
                subword_indices.push(Vec::new());
                continue;
            }

It's fairly unlikely to encounter those ngrams anywhere but we should fix this once we figured out the bug.