Proposed Fix : [RagSequenceForGeneration] generate "without" input_ids 
# What does this PR do?
Proposal of option (2) mentioned in the issue `RagSequenceForGeneration.generate()` without `input_ids`
https://github.com/huggingface/transformers/issues/9184

###  Re-paste here:
In `RagSequenceForGeneration` method `generate()` function, the doc said that both `input_ids` and `context_input_ids` are optional (one of them must be specified) . 

However, in the code https://github.com/ratthachat/transformers/blob/ragseq_context_id/src/transformers/models/rag/modeling_rag.py#L907
It specifically need `input_ids` in all cases.

Not sure which option is the best

(1) simply said `input_ids` is always needed , OR

(2) add code to calculate `nll` if only `context_input_ids` is provided , but in this case `doc_scores` and `context_attention_mask` have to be provided as well (similar to RagModel requirement ) : https://github.com/ratthachat/transformers/blob/ragseq_context_id/src/transformers/models/rag/modeling_rag.py#L588

I think option (2) should be reasonable since  `RagTokenForGeneration` method `generate()` also requires the same.

### who can review?
@lhoestq or @patrickvonplaten 

### Make style applied but failed quality check
I already applied `make style` and passed, but somehow still fails the "code quality check" sorry :( 
@ratthachat - thanks a lot for your PR! I agree that option 2) is the better option and think the general approach is exactly right. I left a couple of statements making that improve readability a bit IMO. Also could we add one test here: 
https://github.com/huggingface/transformers/blob/f38c4ad302dd34faef1137b13e9636f4408b0462/tests/test_modeling_rag.py#L502
called `test_model_generate_from_context_input_ids` that verifies that using `input_ids` and `context_input_ids` yields the same result?<|||||>@ratthachat Don't worry about `make style` I can fix that later!<|||||>Thanks for the review, Patrick! I pretty much agree on every point, esp. the test case. I will do it. :)<|||||>Hi Patrick, I think I addressed every suggestion :)
[ Still fails on code-quality even already applied `make style` again :D ] @patrickvonplaten <|||||>Great job @ratthachat - I'm currently running all RAG slow tests to make sure nothing was unintentionally broken. Will merge once they all pass <|||||>Slow tests all passing<|||||>Nice @patrickvonplaten !