Add Engine event to the follow up requests after checkpointing

[ArEsKay3]

What does this PR do ?

Add an ENGINE_ADD event to the follow up request created when checkpointing. Prevents errors with missing add engine events when calculating ttft on the follow up requests.

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[lmcafee-nvidia]

I feel like there are 2 ways to handle this:

• propagate the event_add_engine object from request to request
• store ttft on the record object instead of the individual request, which I didn't think to do before

I think it's a bit cleaner to store it on the record (or keep it on the first request and remember that the first request is the owner of this event) rather than referencing the same event object on all requests inside the record. What do you think?

[kvareddy]

Let's revisit this later.

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22153787124

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22154719742