drop the mutable default in make_model_instance

[basheer-cloud]

I ran into this while reading through the code path around torchbenchmark/init.py. Drop the mutable default in make_model_instance. Mutable defaults are shared across calls and usually turn into state leaks. I kept the patch small and re-ran syntax checks after applying it.

Backward-compat note: This changes a public function signature by replacing mutable defaults with None guards. Call sites stay source-compatible, but callers introspecting defaults will see None now.

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