sampler : remove "full-context windows" from history-based samplers (#26524)

* Resolve -1 to 1024 instead of ctx-len for samplers

Because of backend-sampling we initialize samplers before the complete
llama_context is there. Therefore, we cannot infer the resolved context
length yet at the time we construct the samplers.

* Shared default of 64 for history-based samplers, remove context_size

tools/server/server-context.cpp

@@ -1807,8 +1807,7 @@ private:
         // initialize samplers
         if (task.need_sampling()) {
             try {
-                slot.smpl.reset(common_sampler_init(
-                        model_tgt, task.params.sampling, (int32_t) llama_n_ctx(ctx_tgt)));
+                slot.smpl.reset(common_sampler_init(model_tgt, task.params.sampling));
             } catch (std::exception & e) {
                 std::string err_msg = std::string("Failed to initialize samplers: ") + e.what();
                 send_error(task, err_msg, ERROR_TYPE_INVALID_REQUEST);
@@ -4148,7 +4147,6 @@ std::unique_ptr<server_res_generator> server_routes::handle_completions_impl(
             task.params = server_schema::eval_llama_cmpl_schema(
                     ctx_server.vocab,
                     params,
-                    meta->slot_n_ctx,
                     meta->logit_bias_eog,
                     data);