Add __all__ to torch/_C/_VariableFunctions.pyi (#40499)
Summary:
Related to https://github.com/pytorch/pytorch/issues/40397
Inspired by ezyang's comment at https://github.com/pytorch/pytorch/issues/40397#issuecomment-648233001, this PR attempts to leverage using `__all__` to explicitly export private functions from `_VariableFunctions.pyi` in order to make `mypy` aware of them after:
```
if False:
from torch._C._VariableFunctions import *
```
The generation of the `__all__` template variable excludes some items from `unsorted_function_hints`, as it seems that those without hints end up not being explicitly included in the `.pyi` file: I leaned on the side of caution and opted for having `__all__` consistent with the definitions inside the file. Additionally, added some pretty-printing to avoid having an extremely long line.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40499
Differential Revision: D22240716
Pulled By: ezyang
fbshipit-source-id: 77718752577a82b1e8715e666a8a2118a9d3a1cf
