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

common/sampling.cpp

@@ -334,15 +334,21 @@ struct common_sampler * common_sampler_init(const struct llama_model * model, st
 }
 
 void common_sampler_free(struct common_sampler * gsmpl) {
-    if (gsmpl) {
-        llama_sampler_free(gsmpl->grmr);
-        llama_sampler_free(gsmpl->chain);
-
-        delete gsmpl;
+    if (!gsmpl) {
+        return;
     }
+
+    llama_sampler_free(gsmpl->grmr);
+    llama_sampler_free(gsmpl->chain);
+
+    delete gsmpl;
 }
 
 void common_sampler_accept(struct common_sampler * gsmpl, llama_token token, bool accept_grammar) {
+    if (!gsmpl) {
+        return;
+    }
+
     const auto tm = gsmpl->tm();
 
     if (gsmpl->grmr && accept_grammar) {
@@ -355,6 +361,10 @@ void common_sampler_accept(struct common_sampler * gsmpl, llama_token token, boo
 }
 
 void common_sampler_reset(struct common_sampler * gsmpl) {
+    if (!gsmpl) {
+        return;
+    }
+
     gsmpl->reset();
 }
 
@@ -415,6 +425,10 @@ void common_perf_print(const struct llama_context * ctx, const struct common_sam
 }
 
 struct llama_sampler * common_sampler_get(const struct common_sampler * gsmpl) {
+    if (!gsmpl) {
+        return nullptr;
+    }
+
     return gsmpl->chain;
 }