Allow to generate several samples for each prompt

[rlouf]

Closes #416

TODO

• Use torch.repeat_interleave so samples of the same sequence in a batch are contiguous (easier for beam search)
• Make sure the FSMs are corrrectly duplicated

Questions

• Do we make the sequence generator return arrays of shape (num_samples * batch_size, num_tokens), and reshape in SequenceGenerator? Reshaping in the generator makes the latter much more complex. We could also not reshape at all, and let the user do it manually, which will simplify chained calls in the future.

[lapp0]

I'm still learning the outlines/generate/ side of the code-base. If there are any components you would like test cases for, that would help me learn :)

IMHO we shouldn't reshape at all. Users should expect an array of results with the same length as their input prompts. A shape-agnostic decoder should be a final step so we aren't required to convert to decodable array and back again in multiple places.