Clarify parser model CPU/GPU code (#4535)
Clarify parser model CPU/GPU code (#4535)

The previous version worked with previous thinc, but only
because some thinc ops happened to have gpu/cpu compatible
implementations. It's better to call the right Ops instance.
honnibal/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,spacy-io/spaCy,explosion/spaCy,spacy-io/spaCy,explosion/spaCy,spacy-io/spaCy,honnibal/spaCy,honnibal/spaCy,honnibal/spaCy,explosion/spaCy,explosion/spaCy,explosion/spaCy,spacy-io/spaCy