Add a line about module owners to labels mapping in CODEOWNERS (#82748)
### Description
We have an issue that maps GitHub labels to owners and people who would like to subscribe to certain discussions. This change hopes to make this issue more discoverable.
### Issue
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### Testing
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Make sure the CODEOWNERS page is still ok
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82748
Approved by: https://github.com/albanD, https://github.com/kit1980, https://github.com/malfet
