Refactor c10::complex and cleanup c10::Scalar

[zasdfgbnm]

Main:

• c10::complex is refactored: it no longer uses inheritance to specialize constructors, but using SFINAE instead. This implementation is cleaner and avoids some compiler bugs.
• c10::Scalar is cleaned up: it no longer needs to store complex as double z[2], c10::complex<double> will work.

Other cleanups:

• numeric_limits of c10::complex is moved to complex_utils.h
• the variable in c10::complex storing real and imag is changed from storage[2] to real_ and imag_
• remove the c10:: before complex when in c10 namespace

[dr-ci]

💊 CI failures summary and remediations

As of commit 90a2372 (more details on the Dr. CI page):

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✅ None of the CI failures appear to be your fault 💚

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• 2/2 broken upstream at merge base 83df3be since May 15

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🚧 2 ongoing upstream failures:

These were probably caused by upstream breakages that are not fixed yet:

• pytorch_linux_xenial_py3_6_gcc5_4_test since May 15
  • 🔁 rerun
• pytorch_linux_bionic_py3_6_clang9_test since May 15
  • 🔁 rerun

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This comment has been revised 1 time.

[anjali411]

nit: can we name them real and imag to keep it similar to the (TBA) python API

[zasdfgbnm]

No, there is already a method named real

[facebook-github-bot]

@anjali411 is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@anjali411 merged this pull request in 928ce29.

[anjali411]

@zasdfgbnm this PR was reverted because it broke master. The OSS ci was based on a branch that was 15 days old.