Fix complex mean and reduction tests not being run (#55640)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55640
Mean is broken for complex types, since #53218 it's now allocating the result
as a real tensor which discards the imaginary component. This wasn't picked up
in testing because `_test_dim_ops` tests are defined as closures inside of
`_test_dim_ops` instead of as methods on the test class. The result is, they
never get run.
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Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D27671127
Pulled By: mruberry
fbshipit-source-id: 4a1f6fea1048919fda7339c867ee78e88f2d7bd2
