[WIP] Refactor sequence generator

[hirofumi0810]

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What does this PR do?

Refactored sequence generator to reuse the function generate_decoder for multi-decoder models. The multi-decoder models will be added after this PR.

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[sravyapopuri388]

The changes seem correct to me but seems like some tests are breaking. Can we update the changes so the tests pass and also verify on some existing models to ensure that the changes are backward compatible

https://github.com/facebookresearch/fairseq/actions/runs/3058436996/jobs/4934645467

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