Make needs_dynamic_casting multiple-complex-type aware.

[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.

We don't actually cast between these complex representations, but the prior implementation would indicate that we needed to dynamic_cast,
because we didn't have mappings for std::complex or thrust::complex.

This PR makes it so they all map to the same dtype.

Note that this has no functional change as all the use sites have already been changed to take this into account.

Differential Revision: D21789694

[dr-ci]

💊 CI failures summary and remediations

As of commit 2d85012 (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 8 times.

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@gchanan merged this pull request in 9b05b1b.

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@gchanan merged this pull request in 9b05b1b.