Add missing header

[raedle]

Summary: Add <functional> header for std::function

Differential Revision: D43021544

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1 Ignored Deployment

| Name | Status | Preview | Comments | Updated | | :--- | :----- | :------ | :------- | :------ | | **playtorch** | ⬜️ Ignored ([Inspect](https://vercel.com/fbopensource/playtorch/DEDVeG2mZ8BGz8EXBrmTefXHGhCE)) | | | Feb 5, 2023 at 6:32PM (UTC) |

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D43021544

[facebook-github-bot]

This pull request has been merged in facebookresearch/playtorch@ea361c5f4b4365ea1729388ad12084c9ac39f146.