Make notebook examples deterministic

[nickfraser]

Some cells have a seed set, but we need to do this everywhere where it's necessary. This would make it much easier to verify them with a script.

[fabianandresgrob]

As torch.manual_seed() is creating a generator, it should be sufficient to set it once at the start of each notebook.
Or does a seed per cell (given that any numbers are generated) have any other advantages?

[nickfraser]

Start of the notebook should be fine, but we shouldn't change the cells where it is set explicitly.