Preserve python backtrace in autograd engine errors. (#43684)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43684
This PR attempts to address #42560 by capturing the appropriate
exception_ptr in the autograd engine and passing it over to the Future.
As part of this change, there is a significant change the Future API where we
now only accept an exception_ptr as part of setError.
For the example in #42560, the exception trace would now look like:
```
> Traceback (most recent call last):
> File "test_autograd.py", line 6914, in test_preserve_backtrace
> Foo.apply(t).sum().backward()
> File "torch/tensor.py", line 214, in backward
> torch.autograd.backward(self, gradient, retain_graph, create_graph)
> File "torch/autograd/__init__.py", line 127, in backward
> allow_unreachable=True) # allow_unreachable flag
> File "torch/autograd/function.py", line 87, in apply
> return self._forward_cls.backward(self, *args)
> File "test_autograd.py", line 6910, in backward
> raise ValueError("something")
> ValueError: something
```
ghstack-source-id: 111109637
Test Plan: waitforbuildbot
Reviewed By: albanD
Differential Revision: D23365408
fbshipit-source-id: 1470c4776ec8053ea92a6ee1663460a3bae6edc5
