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>

src/llama-sampler.h

@@ -15,6 +15,8 @@ struct llama_sampler_chain {
     // has .backend_init() been called?
     bool is_init = false;
 
+    uint32_t n_nodes = 0;
+
     struct info {
         bool is_backend;
 
@@ -33,6 +35,9 @@ struct llama_sampler_chain {
     mutable int32_t n_sample;
 };
 
+uint32_t llama_sampler_backend_n_nodes(const llama_sampler * sampler);
+void llama_sampler_backend_begin(llama_sampler * sampler);
+
 struct llama_sampler * llama_sampler_init_dry_testing(
         float   dry_multiplier,
         float   dry_base,