Update utils.save docs to support path-like / io.Bytes output

[charlesverdad]

The neuralprophet.utils.save function accepts a path:str as the second argument.
This makes me believe that I can only save to a file. However upon looking at the code, it actually
just passes the path to torch.save's f parameter, which accepts io.BytesIO buffer.

Specifically, torch.save supports the type: Union[str, PathLike, BinaryIO, IO[bytes]]

This means it is already possible to use utils.save to write to memory. But the typehints and doc doesn't state that.

[SimonWittner]

Hi @charlesverdad,
thanks for raising this! We'll change that. If you want, you could also create a PR for this.