Fix library top-level imports

[rlouf]

Users are currently running into install issues. After a clean install of outlines they get an error message that asks for transformers to be installed. This should not be the case, as the library is not required for every integration. In this PR we remove transformers and datasets top-level imports, and add per-integration optional dependencies.

TODO

• Test import outlines from clean install
• Test installing outlines with vLLM optional dependencies
• Test installing outlines with MLX optional dependencies
• Test installing outlines with transformers optional dependencies
• Test installing outlines with llama-cpp optional dependencies
• Test installing outlines with exllamav2 optional dependencies
• Test installing outlines with openai optional dependencies
• Update the documentation

Supersedes #1295. Fixes #1263.