[Cherry Pick | Pipeline Parallel] Merge PaddleFleet Pipeline Parallel (LayerSpec and NoPipelineParallel) into Paddle (#78347) * feat(pipeline parallel): merge PaddleFleet PP (LayerSpec and NoPipelineParallel) (#77812) * [Pipeline Parallel] Fix for "Merge PaddleFleet Pipeline Parallel (LayerSpec and NoPipelineParallel) into Paddle #77812" (#78229) * fix(pp): remove assert num_stages when num_virtual_pipeline_stages * refactor(pp_utils): modify gradient extraction logic

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