Move to backend sampling for MTP draft path (#23287)

* Move to backend sampling for MTP draft path

Run top_k(10) on the draft backend. D2H transfers happen only for the top 10 logits

Make backend sampling more robust and fallback to CPU on failure cases, such as with "-sm tensor" or when a backend doesn't support TOP_K.

* Allow sampler chains to be partially offloaded to backend

* Add --spec-draft-backend-sampling argument. Enabled by default.

common/common.h

@@ -305,6 +305,8 @@ struct common_params_speculative_draft {
     float p_split = 0.1f; // speculative decoding split probability
     float p_min   = 0.0f; // minimum speculative decoding probability (greedy)
 
+    bool backend_sampling = true; // offload draft sampling to the backend (default: on)
+
     common_params_model mparams;
 
     llama_context * ctx_tgt = nullptr;