[misc] support export ollama modelfile (#6899)

* support export ollama modelfile

* update config

* add system and num ctx

Former-commit-id: 8c2af7466f4015f300b51841db11bcd2505ebf20

src/llamafactory/hparams/model_args.py

@@ -87,7 +87,7 @@ class ExportArguments:
         metadata={"help": "Path to the directory to save the exported model."},
     )
     export_size: int = field(
-        default=1,
+        default=5,
         metadata={"help": "The file shard size (in GB) of the exported model."},
     )
     export_device: Literal["cpu", "auto"] = field(