Enable torch.autograd typechecks

[malfet]

To help with further typing, move dynamically added native contributions from torch.autograd to torch._C._autograd
Fix invalid error handling pattern in

pytorch/torch/csrc/autograd/init.cpp

Lines 13 to 15 in 89ac30a

	auto tensor_module = THPObjectPtr(PyImport_ImportModule("torch.tensor"));
	if (!tensor_module)
	throw python_error();

PyImport_ImportModule already raises Python exception and nullptr should be returned to properly propagate the to Python runtime.

And all native methods/types in torch/autograd/__init.py after torch._C._init_autograd() has been called
Use f-strings instead of .format in test_type_hints.py
Fixes #44450

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[dr-ci]

💊 CI failures summary and remediations

As of commit 20e76c5 (more details on the Dr. CI page):

---

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

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2 failures confirmed as flaky and can be ignored:

• pytorch_windows_vs2019_py36_cuda11.0_build
• pytorch_windows_vs2019_py36_cuda10.1_build

---

ci.pytorch.org: 1 failed

• Failed: pr/pytorch-linux-bionic-rocm3.7-py3.6

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[albanD]

I'm not convinced this helps for code readability...
Maybe just name one grad_tensors and the other grad_tensors_filled/populated ?

Also the result type of _make_grads is the same no? Why can't you just re-use the same variable as before?

[malfet]

That's a good suggestion, let me rename just that.
And the types are not exactly the same:
grad_tensors is constructed as list of nullable tensors, but _make_grads returns a tuple.
But let me change code a bit to actually construct tuple here as well.

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@malfet has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

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@malfet merged this pull request in 4bead64.