The torch.nn.functional is a Python module that has no “automatic” equivalent in C++.
That said, I don’t think there is a systematic reason native_functions.yaml (the origin for these) doesn’t declare default values but rather historical reasons, especially for not-as-often used functions that originate from torch7 (e.g. other loss functions do have defaults).
If you are adventurous, you could try to provide a PR providing the defaults matching the Python API and see what people say.
Best regards
Thomas