llama: use FA + max. GPU layers by default (#15434)

* llama: use max. GPU layers by default, auto -fa

* ggml-backend: abort instead of segfault

include/llama.h

@@ -179,6 +179,14 @@ extern "C" {
         LLAMA_ATTENTION_TYPE_NON_CAUSAL  = 1,
     };
 
+    enum llama_flash_attn_type {
+        LLAMA_FLASH_ATTN_TYPE_AUTO     = -1,
+        LLAMA_FLASH_ATTN_TYPE_DISABLED = 0,
+        LLAMA_FLASH_ATTN_TYPE_ENABLED  = 1,
+    };
+
+    LLAMA_API const char * llama_flash_attn_type_name(enum llama_flash_attn_type flash_attn_type);
+
     enum llama_split_mode {
         LLAMA_SPLIT_MODE_NONE  = 0, // single GPU
         LLAMA_SPLIT_MODE_LAYER = 1, // split layers and KV across GPUs
@@ -303,6 +311,7 @@ extern "C" {
         enum llama_rope_scaling_type rope_scaling_type; // RoPE scaling type, from `enum llama_rope_scaling_type`
         enum llama_pooling_type      pooling_type;      // whether to pool (sum) embedding results by sequence id
         enum llama_attention_type    attention_type;    // attention type to use for embeddings
+        enum llama_flash_attn_type   flash_attn_type;   // when to enable Flash Attention
 
         // ref: https://github.com/ggml-org/llama.cpp/pull/2054
         float    rope_freq_base;   // RoPE base frequency, 0 = from model
@@ -329,7 +338,6 @@ extern "C" {
         // Keep the booleans together and at the end of the struct to avoid misalignment during copy-by-value.
         bool embeddings;  // if true, extract embeddings (together with logits)
         bool offload_kqv; // offload the KQV ops (including the KV cache) to GPU
-        bool flash_attn;  // use flash attention [EXPERIMENTAL]
         bool no_perf;     // measure performance timings
         bool op_offload;  // offload host tensor operations to device
         bool swa_full;    // use full-size SWA cache (https://github.com/ggml-org/llama.cpp/pull/13194#issuecomment-2868343055)