[torchgen] Prettify generated type annotations (#95877)
Changes:
1. Use class inheritance for `torch/return_types.pyi`:
Before:
```python
max = NamedTuple("max", [("values", Tensor), ("indices", Tensor)])
```
After:
```python
class max(NamedTuple):
values: Tensor
indices: Tensor
```
------
2. Add missing spaces in generated type annotations.
1. Always has a space after `,`.
2. If an argument is annotated, then there need spaces around `=` when it has a default value.
```diff
- def func(..., out: Optional[Tensor]=None, ...) -> Tensor:
+ def func(..., out: Optional[Tensor] = None, ...) -> Tensor:
```
3. If an argument is not annotated, then there should be no spaces around `=` when it has a default value.
```python
def contiguous(self, memory_format=torch.contiguous_format) -> Tensor: ...
```
------
3. ~Remove redundant import alias in `torch/nn/functional.pyi`:~ (Reverted)
UPDATE: `mypy` needs the alias to work.
Before:
```python
from .. import conv1d as conv1d
from .. import conv2d as conv2d
from .. import conv3d as conv3d
from .. import conv_transpose1d as conv_transpose1d
from .. import conv_transpose2d as conv_transpose2d
from .. import conv_transpose3d as conv_transpose3d
from .. import conv_tbc as conv_tbc
from .. import avg_pool1d as avg_pool1d
from .. import relu_ as relu_
from .. import selu_ as selu_
from .. import celu_ as celu_
from .. import rrelu_ as rrelu_
from .. import pixel_shuffle as pixel_shuffle
from .. import pixel_unshuffle as pixel_unshuffle
from .. import channel_shuffle as channel_shuffle
from .. import native_channel_shuffle as native_channel_shuffle
from .. import pdist as pdist
from .. import cosine_similarity as cosine_similarity
```
After:
```python
from .. import (
conv1d,
conv2d,
conv3d,
conv_transpose1d,
conv_transpose2d,
conv_transpose3d,
conv_tbc,
avg_pool1d,
relu_,
selu_,
celu_,
rrelu_,
pixel_shuffle,
pixel_unshuffle,
channel_shuffle,
native_channel_shuffle,
pdist,
cosine_similarity,
)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95877
Approved by: https://github.com/ezyang
