[RAG] Add Ray implementation for distributed retrieval (#9197)

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* uncomment

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* updates

* add docstring

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* fix arg

* fixes

* add unit tests

* update readme

* update readme

* update finetune script

* update test

* add test

* add ray to test dependencies

* separate ray and ray tune

* formatting

* shutdown ray at end of test

* fix tests

* formatting

* formatting

* even more formatting

* address comments

* formatting

* add files

* Update examples/research_projects/rag/test_distributed_retriever.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* address comments

* addressing comments

Co-authored-by: Ubuntu <ubuntu@ip-172-31-21-208.us-west-2.compute.internal>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

examples/legacy/pytorch-lightning/requirements.txt

@@ -19,3 +19,4 @@ pytest
 conllu
 sentencepiece != 0.1.92
 protobuf
+ray