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

* wip

* wip

* wip

* wip

* wip

* wip

* wip

* wip

* uncomment

* uncomment

* wip

* updates

* add docstring

* updates

* 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>

src/transformers/models/rag/retrieval_rag.py

@@ -370,9 +370,8 @@ class RagRetriever:
 
     """
 
-    _init_retrieval = True
-
-    def __init__(self, config, question_encoder_tokenizer, generator_tokenizer, index=None):
+    def __init__(self, config, question_encoder_tokenizer, generator_tokenizer, index=None, init_retrieval=True):
+        self._init_retrieval = init_retrieval
         requires_datasets(self)
         requires_faiss(self)
         super().__init__()