Do not define the macro "isnan" (#43242)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43242
This causes "std::isnan" to produce confusing error messages (std::std has not been declared).
Instead, simply let isnan be exposed in the global namespace.
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D23214374
Pulled By: ezyang
fbshipit-source-id: 9615116a980340e36376a20f2e546e4d36839d4b
