llama: model_loader: add TENSOR_READ_LAZY (#27794)

* llama: model_loader: add TENSOR_GET_ROW_LAZY

* add --tensor-read-lazy

* rename to TENSOR_READ_LAZY

* gen docs

* address comments

include/llama.h

@@ -214,6 +214,12 @@ extern "C" {
     LLAMA_API const char * llama_load_mode_name(enum llama_load_mode load_mode);
     LLAMA_API enum llama_load_mode llama_load_mode_from_str(const char * str);
 
+    enum llama_tensor_read_lazy {
+        LLAMA_TENSOR_READ_LAZY_OFF  = 0, // always read the whole tensor up front
+        LLAMA_TENSOR_READ_LAZY_AUTO = 1, // lazy only for marked tensors larger than 4 GiB (requires mmap)
+        LLAMA_TENSOR_READ_LAZY_ON   = 2, // read the rows of tensors marked by the arch on demand (requires mmap)
+    };
+
     enum llama_context_type {
         LLAMA_CONTEXT_TYPE_DEFAULT = 0,
         LLAMA_CONTEXT_TYPE_MTP     = 1,
@@ -315,6 +321,8 @@ extern "C" {
         enum llama_split_mode split_mode; // how to split the model across multiple GPUs
         enum llama_load_mode  load_mode;  // how to load the model
 
+        enum llama_tensor_read_lazy tensor_read_lazy; // on-demand reading of tensors marked by the arch
+
         // the GPU that is used for the entire model when split_mode is LLAMA_SPLIT_MODE_NONE
         int32_t main_gpu;