Remove the custom sampling loop (SequenceGenerator)

[RobinPicard]

Addresses issue #1422

The objective of this PR is to remove the custom sampling loop (and related entities) that are not used anymore or can be replaced with other elements that are already in outlines. We want to make this commit such that it's convenient to revert the change proposed in the future.

What is modified:

• Remove SequenceGenerator (it was already not used anymore)
• Remove SequenceGeneratorAdapter
• Modify the generate.<something> functions to use Generator instead of SequenceGeneratorAdapter
• Remove the generate tests
• Remove the samplers module and associated tests/documentation (not needed anymore as it was used for providing a sampler for the custom loop)

What is not modified:

• The VLLM and ExLlamaV2Model models that have not been refactored yet and thus still expect to interact with SequenceGeneratorAdapter
• The cookbooks

[rlouf]

I see there's a lot mixed in the last commit, but could we keep SequenceGeneratorAdapter for now and delete the rest?

[RobinPicard]

If we remove Sampler too we would still need to modify many files.

[rlouf]

Let's keep them for the moment then, I don't want the branch to be in a broken state even if temporary.

[RobinPicard]

I modified the commit to only remove SequenceGenerator. I created a separate issue for the rest of: #1450. It should be quite straightforward after we have finished refactoring all models and have updated/deprecated the generate.<something> functions.