[ONNX] Fix 1d case flatten export
Fixes #74142
Previous check `dim is not None and end_dim == dim - 2` didn't consider `end_dim` being negative. However the error only occurs when input tensor has rank 1, and the rank is known to symbolic function. So a better fix is to return `input` directly when rank is 1.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74595
Approved by: https://github.com/garymm
