Making Conversation possible to create directly a full conversation
# What does this PR do?

- Currently conversations contain some state (`conversation.history` namely).
- There is no obvious way to create a conversation from pure logs aside from mutating state.
- The actual result is still buggy because `history` is not correctly
  updated by the Conversation object.

Objectives of this PR:

- Enable creation of a Conversation from existing exchanges.
  ```Conversation("Why do you recommend it ?", past_user_inputs=["Can you recommend a book ?"], generated_responses=["I recommend reading the Lord of the Rings."])```
- Keep relatively close to previous code.
- Fix the bug, that simply discarded history if you created a Conversation through mutation of state. (**Could be backward incompat**)
- `history` renamed `_history` + `_index` as it's now treated as a cache variable (namely to prevent recreating tokens of the conversation all the time.

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## Who can review?

@mfuntowicz
@sgugger

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That's really cool! Also pinging @guillaume-be here as he might is the original author of the pipeline :-)<|||||>Also, @Narsil do you if it's possible to have a chat not widget in the inference API for this pipeline? I think it would be really nice to place around Blenderbot and DialoGPT<|||||>> Also, @Narsil do you if it's possible to have a chat not widget in the inference API for this pipeline? I think it would be really nice to place around Blenderbot and DialoGPT

@patrickvonplaten it's in the pipes, but I've not yet created the widget for huggingface.co, the `api-inference` is ready though.

@patrickvonplaten, @sgugger can you please re-review. There sort of major bug, where we used

`tokenizer.encode(inputs, add_special_tokens=False)` so that BOS end EOS were **not** added on models that required them (instead EOS was added "manually" by the pipeline, leading to poor results on Blenderbot for instance).

Ping @mfuntowicz to make sure we can safely remove that or if there was a strong reason for bypassing tokenizer logic there.<|||||>Also changed the tokenizer behavior to use a real one.<|||||>Thanks for looping me in! It looks like there are a lot of changes, a few comments on my side:
- regarding the change from 
```
inputs = self.tokenizer(inputs, add_special_tokens=False, padding=False).get("input_ids", [])
for input in inputs:
   input.append(self.tokenizer.eos_token_id)
```
to:
```
inputs = self.tokenizer(inputs, **kwargs).get("input_ids", [])
```
are you sure that the behaviour remains correct for DialoGPT? As far as I know DialoGPT uses the GPT2 tokenizer that does not add a `eos` automatically at the end of the encoded input. Test for BlenderBot were added in https://github.com/huggingface/transformers/blob/74f6f91a9dc944b1f8872a0d22abd60050aa41bc/tests/test_pipelines_conversational.py#L102 and I did not observe a poor performance back then - did something change? Also note that BlenderBot does not seem to require a BOS token (https://github.com/huggingface/transformers/blob/f33a6f34461fea61b579a7ec732fcd174b2b41cd/src/transformers/models/blenderbot/tokenization_blenderbot.py#L57)
- The `if len(new_input) > max_length - self.min_length_for_response` was set-up to allow the history to leave some space for future responses. Is this now done as part of the history further capabilities?
- Could you please clarify the need for `_get_history` instead of accessing the history directly?
- Regarding the title of the PR, if you are interested I added this feature to the Rust version of this pipeline a few months ago. The approach seems simpler than the changes proposed here, am I missing something? See https://github.com/guillaume-be/rust-bert/blob/7890d2daffea8e2c792a2e8930294e403b2321dd/src/pipelines/conversation.rs#L416 for reference (I see from your activity that you are familiar with Rust!)

Thanks!<|||||>Hi @guillaume-be ,

Those changes do not belong in this PR anyway, I'll make a separate PR following this one, we should continue the discussion over there.<|||||>It seems the tests are failing in `master` since this merge: https://app.circleci.com/pipelines/github/huggingface/transformers/18333/workflows/72042bfe-4d42-42de-8389-bc0d1cc5494c/jobs/148896<|||||>Yes, was missing a rebase before test, another commit introduced a new warning, which broke the test.

I am not  sure what's the strategy concerning warnings and tests. I've tried to be conservative (meaning explicitly testing them), but I know it might become cumbersome at some point, I can remove those checks if needed.