Add batch inferencing support for GPT2LMHeadModel
# What does this PR do?
This adds correct (absolute) positional embedding to the output, when given attention mask. The positional embedding is calculated using attention mask.
Fixes #3021 
Here is an example usage:
```python
from transformers import GPT2Tokenizer, GPT2LMHeadModel
tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
model = GPT2LMHeadModel.from_pretrained('gpt2', return_dict=True)

# when generating, we will use the logits of right-most token to predict the next token
# so the padding should be on the left
tokenizer.padding_side = "left" 
tokenizer.pad_token = tokenizer.eos_token # to avoid an error

sentences = ["Hello, my dog is a little",
            "Hello, my dog is", # use different length sentences to test batching
            ]
inputs = tokenizer(sentences, return_tensors="pt", padding=True)


output_sequences = model.generate(
    input_ids=inputs['input_ids'],
    attention_mask=inputs['attention_mask'],
    do_sample=False, # disable sampling to test if batching affects output
)

for i in range(len(sentences)):
    print(tokenizer.decode(output_sequences[i]))
    # you can use skip_special_tokens=True in decode() to remove padding token
    # but note that it will also remove other special_tokens
```
outputs:
```
Hello, my dog is a little bit of a mess. I'm not sure if he's going
<|endoftext|><|endoftext|>Hello, my dog is a little bit of a mess. I'm not sure if he
```

comment:
* I think this should be used in `examples/text-generation/run_generation.py`, but I don't know much about other models, and it (code) would be weird if only gpt2 supports batch inferencing.



 albert, bert, GPT2, XLM: @LysandreJik 
 TextGeneration: @TevenLeScao 
 documentation: @sgugger
@patrickvonplaten 
This enables significantly faster generation. 
Here is a simple test I ran.
|                 | generate 20 tokens | generate 100 tokens |
|-----------------|---------------|----------------|
| batch size = 1  | 45.2 s        | 3min 42s       |
| batch size = 32 | 2.25 s (20x)  | 8.36 s (26.5x) |

```python
# following above code
data = sentences * 128 # total 256 sentences
model.cuda();
data = [' '.join([x]*10) for x in data] # make the prompt longer to be more realistic
from tqdm.auto import tqdm

def test(batchsize = 1, max_gen_len = 20):
    for i in tqdm(range(0, len(data), batchsize)):
        batch = data[i: i+batchsize]
        inputs = tokenizer(batch, return_tensors="pt", padding=True)

        output_sequences = model.generate(
            input_ids=inputs['input_ids'].to(model.device),
            attention_mask=inputs['attention_mask'].to(model.device),
            do_sample=False, # disable sampling to test if batching affects output
            pad_token_id=tokenizer.eos_token_id,
            max_length=len(inputs['input_ids'][0]) + max_gen_len, # let it generate longer
        )
        outputs = [tokenizer.decode(x) for x in output_sequences]


%time test(1, 20)

%time test(32, 20)

%time test(1, 100)

%time test(32, 100)
```
<|||||>Hey @cccntu - this is a great addition! I very much like your appraoch here. 
I also checked that all GPT2 SLOW tests function correctly and added a test to make sure batch generation works as expected!

With the current implementation, the user would not be able to define his own `position_ids` for generate, since they are always overwritten in the `prepare_input_ids_for_generation`, but I think this is OK because:
1) Previously, it was impossible for the user to use `position_ids` because they would have to be extended by 1 each generation step - a feature which is not implemented
2) I don't see any reason why position_ids should be different from the way it is implement in the PR right now

@LysandreJik - this feature was heavily requested by the community (linked a couple of issues below) and I think this is a great way to handle GPT2 batch generation. What do you think?<|||||>Related issues: https://github.com/huggingface/transformers/issues/6742, https://github.com/huggingface/transformers/issues/4746,
https://github.com/huggingface/transformers/issues/4824

<|||||>@cccntu - Great work on this PR! If this PR is merged and you want to help the community a tiny bit more, you could give a short description (similar to what you've done above) on how to do batch generation with GPT2 here: https://discuss.huggingface.co/t/batch-generation-with-gpt2/1517. Many people have been asking for this so they would be very glad to see a short forum post about it. 

Thanks a lot again! <|||||>Awesome, great work @cccntu ! It would be amazing if you could write a little description of how your PR works on the forum: https://discuss.huggingface.co/t/batch-generation-with-gpt2/1517 - the community would be very thankful I think :-) <|||||>@patrickvonplaten Thanks for the suggestions! I just added some description to the forum post. 😃 

link to the post for future reference: https://discuss.huggingface.co/t/batch-generation-with-gpt2/1517/2<|||||>Can you please add batch inferencing for GPT2DoubleHeadsModel too?<|||||>@patrickvonplaten @cccntu 

I can see how batch generation is now available. I was wondering, if there's already a way to do the same but with different arguments of `max_len` & `min_length` per encoded_text in a batch in `model.generate()`. Goal here is to generate new text for a batch of encoded text with variable size.<|||||>Hi @spate141, 

Did you mean passing a `max_len` & `min_length` as n-element array?
It would fail here: https://github.com/huggingface/transformers/blob/121dd4332b7e44932b0fbe2fa18bc9fa0131402c/src/transformers/generation_utils.py#L289
Actually, the main issue is here: https://github.com/huggingface/transformers/blob/121dd4332b7e44932b0fbe2fa18bc9fa0131402c/src/transformers/generation_utils.py#L539
We need the right-most logits not be padding, and without modifying `generation_utils.py`, we need to use left-padding, and consequently we need this PR to make sure the positional embedding is correct.

You can also checkout the discussions in #3021, or the forum post: https://discuss.huggingface.co/t/batch-generation-with-gpt2/1517/3 
<|||||>> Did you mean passing a `max_len` & `min_length` as n-element array?
- Yes, exactly! Instead of single int values for all texts in a batch... an array of values for each text in a batch.

I saw the code and I can see why it will fail. https://github.com/huggingface/transformers/issues/3021 seems informative, I'll take a look.

#### Meanwhile I found this way to get what I mentioned:
- Let's assume a model accepts input of `max_len = 64` and we want to generate new text for a piece of text of size 300 tokens. 
- Since we know what's the `max_len` is, we have make sure that we split our input text into 5 batches: `[64, 60, 58, 50, 56, 12]`. 
    - This was done in some clever way to ensure that each text segment follows valid grammar rule and also don't go above that `max_len` limit. 
- For all these 6 text segments we want to generate new text with following min, max values:
    - min_values: `[100, 100, 100, 100, 100, 25]`
    - max_values: `[120, 120, 120, 120, 120, 50]`
- To do that, I can just pass a global min & max values (i.e. 100, 120 respectively) to `model.generate()` along with a tokenized batch of input text segments. 
    - input_ids_shape: `(6, 64)`, min_len: `100`, max_len: `120`
- My only issue here is regarding last text segment in a batch of (6, 64) tokenized tensor. Ideally, we want new generated text of size min of 25 tokens and max of 50 tokens. Generating a new text of size 100 tokens from an input of 12 tokens will be gobbledygook. 
- To handle this, I can just take the last segment of generated text that belongs to our last input text; and split the text and discard everything above its ideal original min/max limit, i.e. (25, 50)

OR
- I can just go with doing same but I combine first 5 text segments and generate text on (5, 64) and generate text for the last one (1, 64) in two pass

OR
- I can just generate everything in 6 pass for each 6 text segments and pass their ideal individual min/max limits

@cccntu In your 2nd comment to this pull request, you posted some impressive results on why doing batch_generation is ideal, specially let's say when you have a GPU. I'm just trying to figure out if doing the same in my case is worth the latency when I have to do some post-processing. I'll post some latency results once I have this setup ready.
<|||||>**Update:** @cccntu 

I went with my 1st approach where I'm generating text for all texts in a single batch with global min, max values. In most cases where my last text chunk in batch is _smaller_ meaning its min/max values are smaller than rest of text chunks in a same batch; I'm just trimming tokens. Results are impressive so far. Some numbers just in case someone stumble upon this thread in future:

**Fixed size text batches:**
- This shows when passing list of text chunks as single batch tensor Vs passing text chunks as individual in for loop. `max_len`, `min_len` variables are kept same in both. Y-axis shows total time in seconds for model to finish generating text.
- All the text chunks are of same size.

![image](https://user-images.githubusercontent.com/10580847/109713174-81d48000-7b66-11eb-94a6-d0c3e6ac77b8.png)

**Variable size text batches:**
- Same as above, but here I'm using variable size text chunks.
- For example: `2 Long, 1 Short` means my input is 2 long size texts + 1 short size text. This is to test what happens when I'm generating text for variable size text chunks in a single batch.
- Also to note that I'm trimming generated text for short text chunks in post processing. So, time on Y-axis include that.

![image](https://user-images.githubusercontent.com/10580847/109713189-87ca6100-7b66-11eb-8859-471c6929668d.png)

Overall, batch text generation seems very useful(🎉) despite one has to add some overhead on top to manage some use cases. <|||||>@cccntu Thanks for your great work! I stumbled upon this thread and would like to know:
1. Would this batching mechanism works for GPT-NEO? 
2. Would this batching mechanism works for pipeline inference?
If so, is there any changes or considerations I need to do or know?<|||||>Thanks for the code! I wonder if now I could generate sentences in a batch withother models (BertGeneration, for instance)? Looking forward to your reply!<|||||>@cccntu  Thanks for your code. By using the correct position_id in this case,  we can do batch inference in pytorch model now.

But when we export the gpt2 model to onnx with `GPT2OnnxConfig`

```python
onnx_config = GPT2OnnxConfig(model.config)
## or using past_key_values mode
# onnx_config = GPT2OnnxConfig(model.config, use_past=True)
```

Then the onnx model inputs don't contation position_id but only input_ids nand attention_masks。
So we can't  do correct batch_inference with onnx model now, right？
<|||||>Thank you for the code. I wonder if you have tested whether there is performance drop when using batch generation? Especially when the GPT-2 model is finetuned with right-padded data.