[ONNX] Fix eye export (#47016)
Summary:
Previously did not considered the case were optional `m` is not provided in `torch.eye(n, m)`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47016
Reviewed By: ejguan
Differential Revision: D24735916
Pulled By: bzinodev
fbshipit-source-id: ec9b410fc59f27d77d4ae40cb38a67537abb3cd8
