[ONNX] Add unittest for exporting embedding_bag (#105862)
Issue list:
* Unsupported FX nodes: {'call_function': ['aten.embedding_renorm.default', ~~'aten._embedding_bag_forward_only.default'~~]}.
* aten._embedding_bag.default not captured by test. Hence this test is not reflecting the pattern seen in model from onnxbench. Update: need validation again, unsure if this is still the case.
* `padding_idx` is always emitted for `aten._embedding_bag` and `aten._embedding_bag_forward_only`. This overload is unsupported by Torchlib.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105862
Approved by: https://github.com/justinchuby
