`decoding_method` argument in generate (#40085)
* factor out expand inputs
* callable arg
* improve docs, add test
* Update docs/source/en/generation_strategies.md
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>
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Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>