[dynamo] keep submodule's name for nn.Sequential when unroolling (#94913)
Currently, when unrolling an nn.Sequential, we use an integer to represent its submodule's name. This produces some difficulty in tracking the origin of the parameters in the export path:
```python
model = nn.Sequential(OrderedDict([
('conv1', nn.Conv2d(1,20,5)),
('relu1', nn.ReLU()),
('conv2', nn.Conv2d(20,64,5)),
('relu2', nn.ReLU())
]))
```
Currently, the submodules will have names such as model.0, model.1 instead of model.conv1, model.relu1. This discrepency causes it difficult to track the origin of paramers because they are represented as model.conv1.foo and model.relu1.foo in model.named_parameters().
We replace enumerate() with named_children() to keep submodule's name.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94913
Approved by: https://github.com/jansel
