Avoid defining bogus CPPTypeAndStdComplexToScalarType<void> by using some decltype tricks.

[gchanan]

Stack from ghstack:

• Kill CPPTypeToScalarType. It's now subsumed by CPPTypeAndStdComplexToScalarType. #39263 Kill CPPTypeToScalarType. It's now subsumed by CPPTypeAndStdComplexToScalarType.
• Avoid defining bogus CPPTypeAndStdComplexToScalarType<void> by using some decltype tricks. #39261 Avoid defining bogus CPPTypeAndStdComplexToScalarType by using some decltype tricks.
• Add dynamic_cast asserts to CPU Loops. #39258 Add dynamic_cast asserts to CPU Loops.
• Make needs_dynamic_casting multiple-complex-type aware. #39255 Make needs_dynamic_casting multiple-complex-type aware.
• Loops: Separate out dynamic_casting concerns from complex overloads. #39254 Loops: Separate out dynamic_casting concerns from complex overloads.
• Avoid a TensorIterator/Loops reinterpret_cast in a test. #39246 Avoid a TensorIterator/Loops reinterpret_cast in a test.

Differential Revision: D21790288

[gchanan]

This is the same as #39071, but restacked and including the suggested changes.

[dr-ci]

💊 CI failures summary and remediations

As of commit 2d7aea0 (more details on the Dr. CI page):

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• 1/1 failures possibly* introduced in this PR
  • 1/1 non-CircleCI failure(s)

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ci.pytorch.org: 1 failed

• Failed: pr/caffe2-py3.6-devtoolset7-rocmrpm-centos7-test

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This comment has been revised 5 times.

[albanD]

LGTM

[facebook-github-bot]

@gchanan merged this pull request in ddf6d49.