server : support multi-modal context checkpoints (#19849)

server : support multi-modal context checkpoints (#19849)

* Modify llama-memory-hybrid-iswa.cpp

* Modify llama-memory-recurrent.cpp

* Modify server-common.cpp

* Modify server-common.h

* Modify server-context.cpp

* Modify server-task.h

* Added comment to llama-memory-hybrid-iswa.cpp

* Remove comment from server-context.cpp

* Stylistic fix server-context.cpp

* Fix an issue when seqrm isn't called in server-context.cpp

* cont : alternative impl

* cont : cleanup

* cont : n_tokens -> int64_t

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Co-authored-by: timkhronos <redacted>