Explicit seed setting

[henryre]

Description of proposed changes

Move to setting seeds explicitly. Ambiguous set_seed has caused a few issues, and this reduces our dependency on PyTorch.

Test plan

Updated unit tests, all passing.

Checklist

Need help on these? Just ask!

• I have read the CONTRIBUTING document.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have run tox -e complex and/or tox -e spark if appropriate.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #1454 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1454      +/-   ##
==========================================
- Coverage   97.58%   97.58%   -0.01%     
==========================================
  Files          55       55              
  Lines        2032     2029       -3     
  Branches      334      334              
==========================================
- Hits         1983     1980       -3     
  Misses         22       22              
  Partials       27       27

Impacted Files	Coverage Δ
snorkel/utils/core.py	94.23% <ø> (-0.6%)	⬇️
snorkel/labeling/model/label_model.py	95.76% <100%> (+0.04%)	⬆️