[nullability] Suppress -Wnullable-to-nonnull-conversion errors in caffe2 (#107418)
Summary: Changelog: Suppresses the nullable to nonnull conversion errors from caffe2.
Test Plan:
```
buck2 build //xplat/caffe2:caffe2Apple
```
Differential Revision: D48453395
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107418
Approved by: https://github.com/seemethere
