[BugFix] Make DP work with connector-delayed new requests

[njhill]

The current DP engine logic assumes that the step() function will always perform a forward pass when there are any running and/or waiting requests. With the new async connector changes this might no longer be the case, specifically when there's only requests in WAITING_FOR_REMOTE_KV state.

These changes adjust the step() function to return a bool indicating whether a forward pass ran, and this is used to determine whether a dummy batch needs to run.

The set_forward_context function is also updated to avoid performing an all-reduce of DP metadata when it's not used in the context of a forward-pass. This is a minimally-invasive fix, but we should probably adjust the connector API to not use the forward context.

Thanks to @wseaton for related testing/experimentation and helping to figure out what changes were needed for this.

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[njhill]

can we add the bool to EngineCoreOutputs?

@simon-mo I guess the main reason for not doing that is that the api-server-scaleout PR will change this step() to return a dict of client index -> EngineCoreOutputs, and the bool flag indicating whether a forward pass happened is kind of separate from this.

[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @njhill.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork