Stream save llama context data to file instead of allocating entire buffer upfront (#2488)

* added stream saving context data to file to avoid allocating unnecessary amounts of memory

* generalised copying state data to file or buffer

* added comments explaining how copy_state_data works

* fixed trailing whitespaces

* fixed save load state example

* updated save load state to use public function in llama.cpp

* - restored breakage of the llama_copy_state_data API
- moved new logic for copying llama state data to internal function

* fixed function declaration order

* restored save load state example

* fixed whitepace

* removed unused llama-util.h include

* Apply suggestions from code review

Co-authored-by: slaren <slarengh@gmail.com>

* Apply code review suggestions

Co-authored-by: slaren <slarengh@gmail.com>

---------

Co-authored-by: slaren <slarengh@gmail.com>

llama-util.h

@@ -149,6 +149,46 @@ struct llama_file {
     }
 };
 
+// llama_context_data
+struct llama_data_context {
+    virtual void write(const void * src, size_t size) = 0;
+    virtual size_t get_size_written() = 0;
+    virtual ~llama_data_context() = default;
+};
+
+struct llama_data_buffer_context : llama_data_context {
+    uint8_t* ptr;
+    size_t size_written = 0;
+
+    llama_data_buffer_context(uint8_t * p) : ptr(p) {}
+
+    void write(const void * src, size_t size) override {
+        memcpy(ptr, src, size);
+        ptr += size;
+        size_written += size;
+    }
+
+    size_t get_size_written() override {
+        return size_written;
+    }
+};
+
+struct llama_data_file_context : llama_data_context {
+    llama_file* file;
+    size_t size_written = 0;
+
+    llama_data_file_context(llama_file * f) : file(f) {}
+
+    void write(const void * src, size_t size) override {
+        file->write_raw(src, size);
+        size_written += size;
+    }
+
+    size_t get_size_written() override {
+        return size_written;
+    }
+};
+
 #if defined(_WIN32)
 static std::string llama_format_win_err(DWORD err) {
     LPSTR buf;