llama_sampling_sample with default args is more naively usable

* Batches populated by either llama_batch_get_one or llama_batch_add work with default args
  * Previously get_one could use the default argument
  * Previously add should usually have used the last index where logits[idx] == true
* This hopefully encourages the use of llama_batch_add
  * By giving expected results when using default arguments.
* Believed to work with any currently well behaved program
  * Default arg now works for both cases (previously would give strange results for add case)
  * Any non-negative number is unaffected and behaves as previously
  * Negative arguments were previously invalid.

common/sampling.cpp

@@ -164,7 +164,7 @@ static llama_token llama_sampling_sample_impl(
                   struct llama_sampling_context * ctx_sampling,
                   struct llama_context * ctx_main,
                   struct llama_context * ctx_cfg,
-                  const int idx,
+                  int idx,
                   bool is_resampling) {  // Add a parameter to indicate if we are resampling
     const llama_sampling_params & params = ctx_sampling->params;
 
@@ -173,6 +173,12 @@ static llama_token llama_sampling_sample_impl(
     const float   mirostat_tau    = params.mirostat_tau;
     const float   mirostat_eta    = params.mirostat_eta;
 
+    if (idx == -1) {
+        const int32_t last_idx = llama_get_logits_last_idx(ctx_main);
+        GGML_ASSERT(last_idx >= 0);
+        idx = last_idx;
+    }
+
     std::vector<float> original_logits;
     auto cur_p = llama_sampling_prepare(ctx_sampling, ctx_main, ctx_cfg, idx, !is_resampling, &original_logits);
     if (!is_resampling) {

common/sampling.h

@@ -119,7 +119,7 @@ std::string llama_sampling_order_print(const llama_sampling_params & params);
 //
 // optional:
 //  - ctx_cfg:      context to use for classifier-free guidance
-//  - idx:          sample from llama_get_logits_ith(ctx, idx)
+//  - idx:          sample from llama_get_logits_ith(ctx, idx), -1 for last logit
 //
 // returns:
 //  - token:      sampled token
@@ -129,7 +129,7 @@ llama_token llama_sampling_sample(
         struct llama_sampling_context * ctx_sampling,
         struct llama_context * ctx_main,
         struct llama_context * ctx_cfg,
-        int idx = 0);
+        int idx = -1);
 
 // Prepares and adjusts the set of token candidates for sampling based on penalties, biases, and sampling parameters.
 llama_token_data_array llama_sampling_prepare(

llama.cpp

@@ -15543,7 +15543,7 @@ float * llama_get_logits_ith(struct llama_context * ctx, int32_t i) {
 int32_t llama_get_logits_last_idx(struct llama_context * ctx) {
     llama_synchronize(ctx);
 
-    if (ctx->cparams.n_batch == 1 && ctx->n_outputs == 1) {
+    if ((ctx->cparams.n_batch == 1) && (ctx->n_outputs == 1)) {
         // trivial case, one input/output, return it.
         return 0;
     }