server: fix n_cmpl not skipping processing prompt (#18663)

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 <redacted>