[NVFuser] assert that vectors are the same size in translateSingleWelford
Before, sometimes out_root.size() < in_root.size(), which would result
in a segfault while accessing out_root[i]. If, instead, we just error
out here, an exception will be thrown and then we'll run the fallback
instead of completely erroring out.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77010
Approved by: https://github.com/eellison, https://github.com/jjsjann123
