Include the ".onnxtext" extension in supported serialization format (#…

…6051)

Usage seen in
https://github.com/onnx/onnx-mlir/blob/5ef91c1be51eb578e61d2ae3c05fa821297a8ee2/test/mlir/onnx/parse/fun_model_test.onnxtext.
(onnx/onnx-mlir#2262). I added the `.onnxtext`
extension to be a known extension for the serializer.

---------

Signed-off-by: Justin Chu <justinchuby@users.noreply.github.com>

onnx/__init__.py

@@ -134,7 +134,7 @@
 # Supported model formats that can be loaded from and saved to
 # The literals are formats with built-in support. But we also allow users to
 # register their own formats. So we allow str as well.
-_SupportedFormat = Union[Literal["protobuf", "textproto"], str]
+_SupportedFormat = Union[Literal["protobuf", "textproto", "onnxtxt", "json"], str]
 # Default serialization format
 _DEFAULT_FORMAT = "protobuf"
 

onnx/serialization.py

@@ -171,7 +171,7 @@ class _TextualSerializer(ProtoSerializer):
     """Serialize and deserialize the ONNX textual representation."""
 
     supported_format = "onnxtxt"
-    file_extensions = frozenset({".onnxtxt"})
+    file_extensions = frozenset({".onnxtxt", ".onnxtext"})
 
     def serialize_proto(self, proto: _Proto) -> bytes:
         text = onnx.printer.to_text(proto)  # type: ignore[arg-type]