Move functorch C++ into aten/src/ATen/functorch

[zou3519]

This PR moves functorch C++ code that does not depend on python into aten/src/ATen/functorch. The C++ code that does depend on python (the python bindings as well as torchdim) will go into torch/csrc/functorch, to come later (see #85263 for initial attempt).

[pytorch-bot]

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❌ 5 Failures

As of commit afbe866:

The following jobs have failed:

• linux-bionic-cuda10.2-py3.9-gcc7 / test (default, 1, 4, linux.4xlarge.nvidia.gpu)
• linux-bionic-cuda10.2-py3.9-gcc7 / test (default, 3, 4, linux.4xlarge.nvidia.gpu)
• linux-bionic-cuda10.2-py3.9-gcc7 / test (jit_legacy, 1, 1, linux.4xlarge.nvidia.gpu)
• linux-bionic-cuda10.2-py3.9-gcc7 / test (slow, 1, 2, linux.4xlarge.nvidia.gpu)
• macos-12-py3-arm64 / test (default, 1, 2, macos-m1-12)

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[zou3519]

@pytorchbot merge -f "A previous version of this passed CI (#85282), before all the queueing and CI issues. In addition, the version of this PR before a rebase passed CI, minus the windows queueing issues and some flaky tests. Merging this now to avoid merge conflicts."

[pytorchmergebot]

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[github-actions]

Hey @zou3519.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.

[zou3519]

Note to Diff Train Operator: if this PR hasn't been reverted yet, and we have discovered issues with it while importing the diff train, and there is not an emergency: please let me try to take a look and fix it first because it is difficult to land this PR. I tested this PR internally in D39663712 and it appears to pass everything (modulo flakiness), but it's possible it will need fixing up to pass the internal build.