Don't use 'not defined' in C++ preprocessing

[cyyever]

While not is a valid C++ keyword, MSVC issues the following warnings

C:\actions-runner\_work\pytorch\pytorch\third_party\fbgemm\include\fbgemm\./FloatConversion.h(292): warning C4067: unexpected tokens following preprocessor directive - expected a newline

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[meta-codesync]

@q10 has imported this pull request. If you are a Meta employee, you can view this in D86135907.

[cyyever]

@q10 The failures are unrelated.

[meta-codesync]

@q10 merged this pull request in 32786e8.