[ONNX] Fix get wrong summary of the docstring in `torch.onnx._deprecation.deprecated` (#87194)
The summary of the deprecated function could be multi-line. Therefore the code below:
https://github.com/pytorch/pytorch/blob/9ac2a06acf75538a35751f785d5f509d6127d6cd/torch/onnx/_deprecation.py#L45
should be adjusted to
```python
summary_and_body = docstring.split("\n\n", 1)
```
Otherwise, the multi-line summary will be separated wrongly.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87194
Approved by: https://github.com/justinchuby, https://github.com/BowenBao
