In comm-compute overlap we will have a graph with:
```
def foo(...):
ag = all_gather(...)
hiding_compute = mm(...)
wait(ag)
```
There is no explicit dependency between the hiding compute and the collectives, but we want to add implicit dependencies from wait->hiding_compute, and from hiding_compute->all_gather to preserve overlap.
Additionally, while bucketing, we will merge collective starts and collective waits together. In this case, we will want to treat the two nodes as a single subgraph - each node in the merged set will have the union of all deps in the set.
This pr adds `AugmentedGraphHelper` that adds the apis, and allows querying for dependency with this augmented graph.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/163959
Approved by: https://github.com/v0i0, https://github.com/IvanKobzarev
ghstack dependencies: #163215, #163754
