[Feature]: Add test verifying CompilationMode.STOCK_TORCH_COMPILE behavior

[morrison-turnansky]

🚀 The feature, motivation and pitch

Test verifies compositional, and that it uses whatever "compilation_config.backend" supplies. Original info: #26355 (comment)

Alternatives

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Additional context

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[zou3519]

You might also want to rename the "dynamo_once" thing too -> this is effectively "torch.compile, but compile once". It will respect whatever compilation_config.backend is.