standardise return type of pipelines and nodes

[lalitpagaria]

Is your feature request related to a problem? Please describe.

Currently most of nodes (retriever, reader, generator, summerizer, translator, document_store, preprocessor etc) and pipelines (qa, faq, doc_search, generator, etc) return type is generic dict. Also they have non-standard return type (missing strict type checking) means some return list of docs, some list of docs in dict, and some list of answer in dict.

I have faced this issue while working on #782.

Describe the solution you'd like

Better to standardise return type with pydentic BaseModel to reduce the confusion and having strict type checking. Also user have clarity about what output to expect from node or pipeline.

[tholor]

Yep, already on our roadmap. Will be part of #600. I will post a comment there and close this issue so that we have everything in one place...