llama : add use_direct_io flag for model loading (#18166)

* Adding --direct-io flag for model loading

* Fixing read_raw() calls

* Fixing Windows read_raw_at

* Changing type off_t to size_t for windows and Renaming functions

* disable direct io when mmap is explicitly enabled

* Use read_raw_unsafe when upload_backend is available, not functional on some devices with Vulkan and SYCL

* Fallback to std::fread in case O_DIRECT fails due to bad address

* Windows: remove const keywords and unused functions

* Update src/llama-mmap.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

---------

Co-authored-by: jtischbein <jtischbein@gmail.com>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

src/llama-model-loader.h

@@ -70,6 +70,7 @@ struct llama_model_loader {
     size_t   n_bytes    = 0;
 
     bool use_mmap = false;
+    bool use_direct_io = false;
     bool check_tensors;
     bool no_alloc;
 
@@ -97,6 +98,7 @@ struct llama_model_loader {
         const std::string & fname,
         std::vector<std::string> & splits, // optional, only need if the split does not follow naming scheme
         bool use_mmap,
+        bool use_direct_io,
         bool check_tensors,
         bool no_alloc,
         const llama_model_kv_override * param_overrides_p,