server: fix n_cmpl not skipping processing prompt (#18663)
* server: fix n_cmpl not skipping processing
* fix infinite loop on empty batch
* cont : init child samplers + modify child logic
* cont : cleanup
* cont : improve n_cmpl logic
- launch the parent task first so it finds the slot with best cache
- parent task waits for child tasks to be launched
- when a child task finishes - remove its cache
* cont : remove redundant function
* cont : reduce parent checks
* fix : nullptr task dereference
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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>
