Update the documentation of the scatter_ method with support for reduction methods. (#40962)
Summary:
Follow up to https://github.com/pytorch/pytorch/pull/36447 . Update for https://github.com/pytorch/pytorch/issues/33389.
Also removes unused `unordered_map` include from the CPP file.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40962
Differential Revision: D22376253
Pulled By: ngimel
fbshipit-source-id: 4e7432190e9a847321aec6d6f6634056fa69bdb8
