Update modules/__init__.pyi.in to include Identity (#29114)
Summary:
Running mypy on the following code is throwing an error, `Module has no attribute Identity`:
```
import torch.nn as nn
layer = nn.Identity()
```
Using the following instead does not give an error:
```
import torch
layer = torch.nn.Identity()
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29114
Differential Revision: D18298225
Pulled By: soumith
fbshipit-source-id: b271bf00086876cca8d63ae0cde6cebf69a7051e
