Convert benchmark_ipfn.py to pyre-strict

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This diff fixes a 'Pyre Strict' issue identified by Quality Insight from [Monetization codehub](https://fburl.com/quality/wmkbc0si).

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## Summary:
This diff converts `fbcode/core_stats/balance/parent_balance/benchmarks/benchmark_ipfn.py` from `# pyre-unsafe` to `# pyre-strict`. The file already had proper type annotations for all functions, parameters, and return values, so only the pyre mode directive needed to be changed.

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Reviewed By: wesleytlee

Differential Revision: D87324577

fbshipit-source-id: afd0367eededa1b7602ddeaefb1ac1667501d1e4

Author: talgalili

benchmarks/benchmark_ipfn.py

@@ -3,7 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-# pyre-unsafe
+# pyre-strict
 
 from __future__ import annotations