Fix continuous batching in `transformers serve` (#39149)
* Fix CB
* Nit
* Update src/transformers/commands/serving.py
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>
* Add todos
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Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>
