Fix getAttribute (#31011)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31011
`getAttribute` is supposed to throw when there the attribute is not
found rather than return a `nullptr`.
Test Plan:
.
Imported from OSS
Differential Revision: D18898417
fbshipit-source-id: 0fe7d824b978ad19bb5ef094d3aa560e9fc57f87
