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

examples/batched/batched.cpp

@@ -81,7 +81,6 @@ int main(int argc, char ** argv) {
         sampler_configs.push_back({ i, smpl });
     }
 
-    // TODO: temporarily gated behind a flag
     if (params.sampling.backend_sampling) {
         ctx_params.samplers   = sampler_configs.data();
         ctx_params.n_samplers = sampler_configs.size();