Fix sen_idx for sliced corpus in Extractor

[jumelet]

diagNNose/diagnnose/corpus/corpus.py

Line 166 in 76d9fe1

examples = [ex for ex in self.examples if ex.sen_idx in sen_ids]

When taking a slice that does not start at 0 this leads to a bug in the Extractor which can not handle non-contiguous sen_ids.