Fix nested module example

[matsumotosan]

What does this PR do?

Fixes #20609

• nested lightning modules are not supported but example had nested modules
• only encoder (nn.Module) is needed for feature extractor anyway

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📚 Documentation preview 📚: https://pytorch-lightning--20923.org.readthedocs.build/en/20923/

[Borda]

@lantiga mind have look, pls 🙏