Add UninitializedBuffer to nn docs (#59021)
Summary:
The `UninitializedBuffer` class was previously left out of `nn.rst`, so it was not included in the generated documentation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59021
Reviewed By: anjali411
Differential Revision: D28723044
Pulled By: jbschlosser
fbshipit-source-id: 71e15b0c7fabaf57e8fbdf7fbd09ef2adbdb36ad
