gguf-py : decouple adding metadata from writing in GGUFWriter (#7827)

Main changes of this PR is to consolidate GGUFWriter.add_key and GGUFWriter.add_val into GGUFWriter.add_key_value. 

In addition use_temp_file is now opt-in instead of opt-out defaulting to False.

Also GGUFWriter now does not require output file name until when actually writing to it.

And GGUFWriter doesn't really need to eagerly prepare the data layout of the metadata

gguf-py/scripts/gguf-new-metadata.py

@@ -101,8 +101,7 @@ def copy_with_new_metadata(reader: gguf.GGUFReader, writer: gguf.GGUFWriter, new
             logger.debug(f'Copying {field.name}')
 
         if val.value is not None:
-            writer.add_key(field.name)
-            writer.add_val(val.value, val.type)
+            writer.add_key_value(field.name, val.value, val.type)
 
     if gguf.Keys.Tokenizer.CHAT_TEMPLATE in new_metadata:
         logger.debug('Adding chat template(s)')
@@ -111,8 +110,7 @@ def copy_with_new_metadata(reader: gguf.GGUFReader, writer: gguf.GGUFWriter, new
 
     for key, val in new_metadata.items():
         logger.debug(f'Adding {key}: "{val.value}" {val.description}')
-        writer.add_key(key)
-        writer.add_val(val.value, val.type)
+        writer.add_key_value(key, val.value, val.type)
 
     total_bytes = 0