[profiler] Reintroduce forward-backward links (#102424)
**TL;DR:** This re-introduces links between backward kernels and their corresponding forward kernels.
<img width="1020" alt="Screenshot 2023-05-26 at 7 25 22 PM" src="https://github.com/pytorch/pytorch/assets/5067123/02571b59-859c-4c9e-b3ef-121ef3159812">
In the example above, you can see there are two such flows - one for aten::add, and one for aten::binary_cross_entropy
### Details
Forward/backward links were added in https://github.com/pytorch/pytorch/pull/62553, but then disabled in https://github.com/pytorch/pytorch/pull/72904 due to segfaults (e.g. https://github.com/pytorch/pytorch/issues/69443).
Between now and when the fwd-bwd links were disabled, there's been a lot of refactoring; so this PR updates the implementation:
* Use a raw profiler::impl::Result instead of a KinetoEvent
* Move the implementation to collection.cpp, where the TraceWrapper is currently handled.
* Sort the events before processing, because they aren't always in chronological order
* There can now be more than one event in the backward pass that matches the sequenceNr-threadID pair. The implementation needed to be updated to avoid showing multiple endpoints for a given sequenceNr-threadID pair ([ptr to where the bwd sequenceNr-threadID pair is duplicated](https://github.com/pytorch/pytorch/blob/6e3e3dd477e0fb9768ee890ff6c6e5dd54aca679/torch/csrc/profiler/collection.cpp#L398-L399)).
Next, we need to verify that https://github.com/pytorch/pytorch/issues/69443 is fixed. Running the repro no longer errors. Looking further into the details of the issue it seems like the handling of the [raw linkedActivity pointer (old code from 2021)](https://github.com/pytorch/kineto/blob/6089dcac48329350d162ce9ac24d04f0733396f1/libkineto/src/output_json.cpp#L283) resulted in the segfault. Now, it doesn't look like the linked activity is used anywhere in output_json.cpp so the issue should be fixed.
### Testing
#### 1. unit test
`test_profiler_fwd_bwd_link` was un-skipped. It was modified to match the new implementation.
#### 2. https://github.com/pytorch/pytorch/issues/69443
I ran the repro in https://github.com/pytorch/pytorch/issues/69443 and verified there were no segfaults.
#### 3. Duplicate flow IDs
When forward-backward connections were first introduced, gpu-cpu async links had not been introduced. There's a possibility that gpu-cpu links and fwd-bwd links could interfere if their IDs overlap.
I manually tested this in chrome://tracing; I edited a file so that a gpu-cpu link had the same ID as one of the fwd-bwd connections. The chrome tracing UI continued showing both types of links.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102424
Approved by: https://github.com/aaronenyeshi
