Bring docstring to .pyi file (#114705)

Fixes #37762

Since the original issue hasn't been making progress for more than 3 years, I am attempting to make this PR to at least make some progress forward.

This PR attempts to add docstring to the `.pyi` files. The docstrings are read from [`_torch_docs`](https://github.com/pytorch/pytorch/blob/main/torch/_torch_docs.py) by mocking [`_add_docstr`](9f073ae304/torch/csrc/Module.cpp (L329)), which is the only function used to add docstring.

Luckily, `_torch_docs` has no dependencies for other components of PyTorch, and can be imported without compiling `torch._C` with `_add_docstr` mocked.

The generated `.pyi` file looks something like the following:

[_VariableFunctions.pyi.txt](https://github.com/pytorch/pytorch/files/13494263/_VariableFunctions.pyi.txt)

<img width="787" alt="image" src="https://github.com/pytorch/pytorch/assets/6421097/73c2e884-f06b-4529-8301-0ca0b9de173c">

And the docstring can be picked up by VSCode:

<img width="839" alt="image" src="https://github.com/pytorch/pytorch/assets/6421097/1999dc89-a591-4c7a-80ac-aa3456672af4">

<img width="908" alt="image" src="https://github.com/pytorch/pytorch/assets/6421097/ecf3fa92-9822-4a3d-9263-d224d87ac288">

Pull Request resolved: https://github.com/pytorch/pytorch/pull/114705
Approved by: https://github.com/albanD

torch/_tensor_docs.py

@@ -2,7 +2,7 @@
 
 import torch._C
 from torch._C import _add_docstr as add_docstr
-from ._torch_docs import parse_kwargs, reproducibility_notes
+from torch._torch_docs import parse_kwargs, reproducibility_notes
 
 
 def add_docstr_all(method, docstr):