def backprop(one_d_tokvecs):
            """Callback to actually do the backprop. Passed to last listener."""
            accumulate_gradient(one_d_tokvecs)
            d_docs = bp_tokvecs(d_tokvecs)
            if sgd is not None:
                self.finish_update(sgd)
            return d_docs
Callback to actually do the backprop. Passed to last listener.
spacy/pipeline/tok2vec.py