llama : support multi-output backend sampling (#25532)

* Enable backend sampling with token speculation

* Clamp the mask sum before converting it into the sampled index

* Add a numeric context parameter declaring the maximum outputs one sequence

* More fixes

* Don't reuse memory for output views.

* Match dist between CPU and GPU

* Fix CPU and backend sampling mismatches

* Simpify some of the changes

* Fix tests on Vulkan

* More test fixes

* Rebase changes

* Rebase and address review comments

* Address review comments

* Address review comments

* Update src/llama-sampler.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

---------

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

examples/speculative-simple/speculative-simple.cpp

@@ -5,6 +5,7 @@
 #include "log.h"
 #include "llama.h"
 
+#include <algorithm>
 #include <clocale>
 #include <cstdio>
 #include <cstring>
@@ -29,6 +30,11 @@ int main(int argc, char ** argv) {
         return 1;
     }
 
+    const auto output_limits = common_speculative_get_output_limits(
+            params.n_batch, params.n_parallel, common_speculative_n_max(&params.speculative));
+    params.n_outputs_max = output_limits.total;
+    params.n_outputs_max_per_seq = output_limits.per_seq;
+
     // init llama.cpp
     llama_backend_init();
     llama_numa_init(params.numa);
@@ -55,6 +61,9 @@ int main(int argc, char ** argv) {
 
         auto params_dft = params;
 
+        params_dft.n_outputs_max = params.n_parallel;
+        params_dft.n_outputs_max_per_seq = 1;
+
         params_dft.devices      = params_spec.devices;
         params_dft.model        = params_spec.mparams;
         params_dft.n_gpu_layers = params_spec.n_gpu_layers;