Commit 121dd43 changes DialoGPT generation behavior
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.3.1
- Platform: Linux-4.4.0-127-generic-x86_64-with-debian-stretch-sid
- Python version: 3.7.3
- PyTorch version (GPU?): 1.6.0+cu101 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: yes (1 TITAN-XP)
- Using distributed or parallel set-up in script?: no

### Who can help
@cccntu @patrickvonplaten @LysandreJik 

## Information

Model I am using (Bert, XLNet ...): DialoGPT-large

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Checkout [121dd43](https://github.com/huggingface/transformers/commit/121dd43).

2. Run the DialoGPT "How to use" code given [here](https://huggingface.co/microsoft/DialoGPT-medium), but change `DialoGPT-medium` to `DialoGPT-large`:
```python
from transformers import AutoModelForCausalLM, AutoTokenizer
import torch


tokenizer = AutoTokenizer.from_pretrained("microsoft/DialoGPT-large")
model = AutoModelForCausalLM.from_pretrained("microsoft/DialoGPT-large")

# Let's chat for 5 lines
for step in range(5):
    # encode the new user input, add the eos_token and return a tensor in Pytorch
    new_user_input_ids = tokenizer.encode(input(">> User:") + tokenizer.eos_token, return_tensors='pt')

    # append the new user input tokens to the chat history
    bot_input_ids = torch.cat([chat_history_ids, new_user_input_ids], dim=-1) if step > 0 else new_user_input_ids

    # generated a response while limiting the total chat history to 1000 tokens, 
    chat_history_ids = model.generate(bot_input_ids, max_length=1000, pad_token_id=tokenizer.eos_token_id)

    # pretty print last ouput tokens from bot
    print("DialoGPT: {}".format(tokenizer.decode(chat_history_ids[:, bot_input_ids.shape[-1]:][0], skip_special_tokens=True)))
```

3. For the user's first utterance, type "Hello, how are you?". I get this output:
```
>> User:Hello, how are you?
DialoGPT: 're you a fan of the show?
```

Note: this problem is still present in the current version of master (`5148f43`).

## Expected behavior

With the previous commit, `0c64b18`, I get this output:
```
>> User:Hello, how are you?
DialoGPT: I'm good, you?
```

## Possible cause
The issue seems to be related to the `<|endoftext|>` token, which is used at the end of every utterance. This is being regarded as a padding token, and thus it's attention-masked, which also seems to affect the position ids.
Hi @abisee , sorry for the inconvenience.

Even though you did not pass in attention mask, it is created here: (first 2 lines)
https://github.com/huggingface/transformers/blob/5148f433097915f30864bf0ca6090656fecefbb8/src/transformers/generation_utils.py#L352-L363

changing this
`chat_history_ids = model.generate(bot_input_ids, max_length=1000, pad_token_id=tokenizer.eos_token_id)`
to 
`chat_history_ids = model.generate(bot_input_ids, max_length=1000, )`
seems to solve the problem. (the pad_token_id will still be set to tokenizer.eos_token_id, but after attention_mask is set to all ones)

Here is how the bug can happen: 
If someone tries to 
* use eos_token_id in sentences
* and also sets pad_token_id=eos_token_id
* and attention mask is created like this (using positions of pad_token_id). (there is no problem when using tokenizer to create attention mask)

Don't have a better solution for now, will think about it. 
@patrickvonplaten @LysandreJik What do you think?
Maybe generate() should not create attention mask for users, but this can break other code, too.<|||||>Thanks for the response @cccntu!

My understanding is that both GPT2 and DialoGPT were trained without a pad token; i.e. neither model has a pad token embedding. In that case, why does the DialoGPT example code contain `pad_token_id=tokenizer.eos_token_id`? What's the purpose of doing this, if a pad token isn't needed for generation, and the EOS token was never used as a pad token during training?<|||||>**For generation**, it seems that attention masks are created automatically (if there's an assigned pad token that appears in the input). See `GenerationMixin.generate()`:
```
# create attention mask if necessary
        # TODO (PVP): this should later be handled by the forward fn() in each model in the future see PR 3140
        if (attention_mask is None) and (pad_token_id is not None) and (pad_token_id in input_ids):
            attention_mask = input_ids.ne(pad_token_id).long()
        elif attention_mask is None:
            attention_mask = input_ids.new_ones(input_ids.shape)
```
However **for training** (at least for GPT2 models), as far as I can tell, the attention mask is _not_ created automatically, even if there's an assigned pad token that appears in the input. 

This seems like an unexpected discrepancy, and another reason to put the attention mask creation in the model's `forward` as proposed by @thomwolf in [PR 3140](https://github.com/huggingface/transformers/pull/3140).<|||||>That's a super interesting issue! Thanks for posting it here! 

So in short, in order to be able to do batch_generation with GPT2 (or Beam Search), we have to use some kind of token as the `pad_token_id` in case one batch finishes early. We decided a while back that for GPT2 we will just use the `eos_token_id` as the `pad_token_id` in this case. 

Just as you guys noticed the problem lies in `generate()` automatically creating the `attention_mask` and falsely assuming the `eos_token_id` is a `pad_token_id` . 

IMO, it was a mistake to automatically create the `attention_mask` in `generate()` as it could lead to unexpected problems such as those! 

I'm currently doing a big `generate()` refactor and in this refactor the problem should be solved (see comment in PR linked below).

I hope that I'll be able to merge the PR in ~1 week.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
