Explicitly import functional into the torch.nn namespace
The `torch.nn.functional` module is not currently imported into the `torch.nn` namespace. This causes Pylance, at a minimum, to complain about it not being a known member.
This should not add any overhead because `torch.nn.functional` is being imported implicitly in these two common scenarios, at least.
```py3
import torch
assert "functional" in dir(torch.nn)
```
```py3
from torch import nn
assert "functional" in dir(nn)
```
This change adds an explicit import.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59716
Approved by: https://github.com/jbschlosser
