[kineto] Optimize getStepCallbacks for common case of no active callbacks
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77804
IIUC, the result of this function will be empty and unused if there are no sampled callbacks, which is the common case. We can accelerate this case by wrapping the result in an optional to save initializing an empty SmallVector.
Differential Revision: [D36497279](https://our.internmc.facebook.com/intern/diff/D36497279/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D36497279/)!
Approved by: https://github.com/robieta
