[SR] Give VarStackNodeWrapper an iterator (#69922)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69922
D32596934 (https://github.com/pytorch/pytorch/commit/65f54bc000c4824a4e999ebfb6a27b252b696b0d) made the serial stack implementation a bit brittle. It introduced a new container type: `VarStackNodeWrapper`. This type was used as a template parameter in the serial stack implementation.
The other type used in the serial stack implementation is `at::ArrayRef<at::Tensor>`. Ideally, the interface of `VarStackNodeWrapper` should be as close as possible to this other type. However, because the new container type did not have an iterator, expressions like this would fail to compile:
```
for (const auto& tensor : tensors) {
// do something
}
```
Introducing this iterator will make the code easier to maintain going forward.
Test Plan:
`buck test caffe2/benchmarks/static_runtime:static_runtime_cpptest -- Stack`
I consider this a `VarStack` implementation detail, so I'd prefer not to test it directly. We can test it implicitly by adding some code to the serial stack implementation that uses the iterator.
Reviewed By: swolchok
Differential Revision: D33101489
fbshipit-source-id: 7cf44c072d230c41bd9113cf2393bc6a6645a5b5
