Pass Generator not Sequence for GraphSAGE in hateful-twitter demo (#1019)
Pass Generator not Sequence for GraphSAGE in hateful-twitter demo (#1019)

The GraphSAGE model was changed to take a generator rather than a sequence
(created with `generator.flow`) in #498, but the
`demos/use-cases/hateful-twitter.ipynb` notebook was mistakenly not updated.

See: #912

stellargraph/stellargraph,stellargraph/stellargraph