Followup cleanup in cmake.py and add a comment in setup.py (#21792)
Summary:
Following up b811b6d5c03596d789a33d7891b606842e01f7d2
* Use property instead of __setattr__ in CMake.
* Add a comment clarifying when built_ext.run is called.
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cc ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21792
Differential Revision: D15860606
Pulled By: umanwizard
fbshipit-source-id: ba1fa07f58d4eac81ac27fa9dc7115d1cdd3dec0
