graph : utilize ggml_build_forward_select() to avoid reallocations (#18898)

* graph : avoid branches between embedding and token inputs

* models : make deepstack graphs (e.g. Qwen3 VL) have constant topology

* ci : enable -DGGML_SCHED_NO_REALLOC=ON for server CI

* cont : pad token embeddings to n_embd_inp

src/llama-context.cpp

@@ -2903,7 +2903,7 @@ void llama_context::opt_epoch_iter(
                 };
                 ctx_compute_opt = ggml_init(params);
             }
-            ggml_opt_prepare_alloc(opt_ctx, ctx_compute_opt, gf, res->get_tokens(), res->get_logits());
+            ggml_opt_prepare_alloc(opt_ctx, ctx_compute_opt, gf, res->get_inp_tokens(), res->get_logits());
             ggml_opt_alloc(opt_ctx, train);
 
             res->set_inputs(&ubatch);