Add GPU option for spaCy preprocessor

[henryre]

Description of proposed changes

Credit to @rjurney. Adds optional GPU mode for spaCy preprocessor.

Test plan

Previously tested by @rjurney, plus existing tests

[vincentschen]

after tests pass, ofc

[codecov[bot]]

Codecov Report

Merging #1523 into master will decrease coverage by 0.09%. The diff coverage is 60%.

@@            Coverage Diff            @@
##           master    #1523     +/-   ##
=========================================
- Coverage    97.6%   97.51%   -0.1%     
=========================================
  Files          55       55             
  Lines        2047     2052      +5     
  Branches      337      338      +1     
=========================================
+ Hits         1998     2001      +3     
- Misses         22       23      +1     
- Partials       27       28      +1

Impacted Files	Coverage Δ
snorkel/labeling/lf/nlp.py	100% <100%> (ø)	:arrow_up:
snorkel/preprocess/nlp.py	86.66% <33.33%> (-13.34%)	:arrow_down: