context : reserve new scheduler when graph topology changes (#18547)

* context : reserve new scheduler when graph topology changes

* cont : fix

* cont : fix reserve

* cont : reserve only when changes occur + timing

* context : add comments

* llama : reserve on sampler changes

* common : allow null common_sampler

* server : task declares needs (embd, logits, sampling)

* server : do not init sampler if not needed

* llama : fix need_reserve when unsetting a sampler

* server : consolidate slot reset/clear logic

tools/server/server-task.h

@@ -156,6 +156,36 @@ struct server_task {
         return tokens.size();
     }
 
+    bool need_embd() const {
+        switch (type) {
+            case SERVER_TASK_TYPE_EMBEDDING:
+            case SERVER_TASK_TYPE_RERANK:
+                return true;
+            default:
+                return false;
+        }
+    }
+
+    bool need_logits() const {
+        switch (type) {
+            case SERVER_TASK_TYPE_COMPLETION:
+            case SERVER_TASK_TYPE_INFILL:
+                return true;
+            default:
+                return false;
+        }
+    }
+
+    bool need_sampling() const {
+        switch (type) {
+            case SERVER_TASK_TYPE_COMPLETION:
+            case SERVER_TASK_TYPE_INFILL:
+                return true;
+            default:
+                return false;
+        }
+    }
+
     static task_params params_from_json_cmpl(
         const llama_vocab * vocab,
         const common_params & params_base,