Use doctest directly to get docstring examples (#50596)

Summary:
This PR addresses [a two-year-old TODO in `test/test_type_hints.py`](12942ea52b/test/test_type_hints.py (L21-L22)) by replacing most of the body of our custom `get_examples_from_docstring` function with [a function from Python's built-in `doctest.DocTestParser` class](https://docs.python.org/3/library/doctest.html#doctest.DocTestParser.get_examples). This mostly made the parser more strict, catching a few errors in existing doctests:

- missing `...` in multiline statements
- missing space after `>>>`
- unmatched closing parenthesis

Also, as shown by [the resulting diff of the untracked `test/generated_type_hints_smoketest.py` file](https://pastebin.com/vC5Wz6M0) (also linked from the test plan below), this introduces a few incidental changes as well:

- standalone comments are no longer preserved
- indentation is now visually correct
- [`example_torch_promote_types`](4da9ceb743/torch/_torch_docs.py (L6753-L6772)) is now present
- an example called `example_torch_tensor___array_priority__` is added, although I can't tell where it comes from
- the last nine lines of code from [`example_torch_tensor_align_as`](5d45140d68/torch/_tensor_docs.py (L386-L431)) are now present
- the previously-misformatted third line from [`example_torch_tensor_stride`](5d45140d68/torch/_tensor_docs.py (L3508-L3532)) is now present

Pull Request resolved: https://github.com/pytorch/pytorch/pull/50596

Test Plan:
Checkout the base commit, typecheck the doctests, and save the generated file:
```
$ python test/test_type_hints.py TestTypeHints.test_doc_examples
$ cp test/generated_type_hints_smoketest.py /tmp
```
Then checkout this PR, do the same thing, and compare:
```
$ python test/test_type_hints.py TestTypeHints.test_doc_examples
$ git diff --no-index {/tmp,test}/generated_type_hints_smoketest.py
```
The test should succeed, and the diff should match [this paste](https://pastebin.com/vC5Wz6M0).

Reviewed By: walterddr

Differential Revision: D25926245

Pulled By: samestep

fbshipit-source-id: 23bc379ff438420e556263c19582dba06d8e42ec

torch/serialization.py

@@ -566,7 +566,7 @@ def load(f, map_location=None, pickle_module=pickle, **pickle_load_args):
         >>> torch.load('tensors.pt', map_location={'cuda:1':'cuda:0'})
         # Load tensor from io.BytesIO object
         >>> with open('tensor.pt', 'rb') as f:
-                buffer = io.BytesIO(f.read())
+        ...     buffer = io.BytesIO(f.read())
         >>> torch.load(buffer)
         # Load a module with 'ascii' encoding for unpickling
         >>> torch.load('module.pt', encoding='ascii')