Transformers Encoder and Decoder Inference
Author: anandhperumal
Pull request: no

## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....):

Language I am using the model on (English, Chinese....):

The problem arise when using:
* [X ] the official example scripts: (give details)
* [ ] my own modified scripts: (give details)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ X] my own task or dataset: (give details)

## To Reproduce

Steps to reproduce the behavior:

1. Error while doing inference.
```
from transformers import PreTrainedEncoderDecoder, BertTokenizer
model = PreTrainedEncoderDecoder.from_pretrained('bert-base-uncased',''bert-base-uncased')
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
encoder_input_ids=tokenizer.encode("Hi How are you")
import torch
ouput = model(torch.tensor( encoder_input_ids).unsqueeze(0))
```
and the error is 
```
TypeError: forward() missing 1 required positional argument: 'decoder_input_ids'
```
During inference why is decoder input is expected ?

Let me know if I'm missing anything?
## Environment

OS: ubuntu
Python version: 3.6
PyTorch version:1.3.0
PyTorch Transformers version (or branch):2.2.0
Using GPU ? Yes
Distributed of parallel setup ? No
Any other relevant information:


## Additional context

<!-- Add any other context about the problem here. -->


Comments:
As said in #2117 by @rlouf (an author of Transformers), **at the moment** you can use `PreTrainedEncoderDecoder` with only **BERT** model both as encoder and decoder.

In more details, he said: "_Indeed, as I specified in the article, PreTrainedEncoderDecoder only works with BERT as an encoder and BERT as a decoder. GPT2 shouldn't take too much work to adapt, but we haven't had the time to do it yet. Try PreTrainedEncoderDecoder.from_pretrained('bert-base-uncased', 'bert-base-uncased') should work. Let me know if it doesn't._".

> ##  Bug
> Model I am using (Bert, XLNet....):
> 
> Language I am using the model on (English, Chinese....):
> 
> The problem arise when using:
> 
> * [X ] the official example scripts: (give details)
> * [ ]  my own modified scripts: (give details)
> 
> The tasks I am working on is:
> 
> * [ ]  an official GLUE/SQUaD task: (give the name)
> * [ X] my own task or dataset: (give details)
> 
> ## To Reproduce
> Steps to reproduce the behavior:
> 
> 1. Error while doing inference.
> 
> ```
> from transformers import PreTrainedEncoderDecoder, BertTokenizer
> model = PreTrainedEncoderDecoder.from_pretrained('bert-base-uncased','gpt2')
> tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
> encoder_input_ids=tokenizer.encode("Hi How are you")
> import torch
> ouput = model(torch.tensor( encoder_input_ids).unsqueeze(0))
> ```
> 
> and the error is
> 
> ```
> TypeError: forward() missing 1 required positional argument: 'decoder_input_ids'
> ```
> 
> During inference why is decoder input is expected ?
> 
> Let me know if I'm missing anything?
> 
> ## Environment
> OS: ubuntu
> Python version: 3.6
> PyTorch version:1.3.0
> PyTorch Transformers version (or branch):2.2.0
> Using GPU ? Yes
> Distributed of parallel setup ? No
> Any other relevant information:
> 
> ## Additional context

@TheEdoardo93 it doesn't matter whether it is GPT2 or bert. Both has the same error :
I'm trying to play with GPT2 that's why I pasted my own code.

Using BERT as an Encoder and Decoder
```
>>> model = PreTrainedEncoderDecoder.from_pretrained('bert-base-uncased','bert-base-uncased')
>>> ouput = model(torch.tensor( encoder_input_ids).unsqueeze(0))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/guest_1/.local/lib/python3.6/site-packages/torch/nn/modules/module.py", line 541, in __call__
    result = self.forward(*input, **kwargs)
TypeError: forward() missing 1 required positional argument: 'decoder_input_ids'
```

First of all, authors of Transformers are working on the implementation of `PreTrainedEncoderDecoder` object, so it's not a definitive implementation, e.g. the code lacks of the implementation of some methods. Said so, I've tested your code and I've revealed how to working with `PreTrainedEncoderDecoder` **correctly without bugs**. You can see my code below.

In brief, your problem occurs because you have not passed _all_ arguments necessary to the `forward` method. By looking at the source code [here](https://github.com/huggingface/transformers/blob/master/transformers/modeling_encoder_decoder.py), you can see that this method accepts **two** parameters: `encoder_input_ids` and `decoder_input_ids`. In your code, you've passed _only one_ parameter, and the Python interpreter associates your `encoder_input_ids` to the `encoder_input_ids` of the `forward` method, but you don't have supply a value for `decoder_input_ids` of the `forward` method, and this is the cause that raise the error.

```
Python 3.6.9 |Anaconda, Inc.| (default, Jul 30 2019, 19:07:31) 
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import transformers
/home/<user>/anaconda3/envs/huggingface/lib/python3.6/site-packages/tensorboard/compat/tensorflow_stub/dtypes.py:541: FutureWarning: Passing (type, 1) or '1type' as a synonym of type is deprecated; in a future version of numpy, it will be understood as (type, (1,)) / '(1,)type'.
  _np_qint8 = np.dtype([("qint8", np.int8, 1)])
/home/<user>/anaconda3/envs/huggingface/lib/python3.6/site-packages/tensorboard/compat/tensorflow_stub/dtypes.py:542: FutureWarning: Passing (type, 1) or '1type' as a synonym of type is deprecated; in a future version of numpy, it will be understood as (type, (1,)) / '(1,)type'.
  _np_quint8 = np.dtype([("quint8", np.uint8, 1)])
/home/<user>/anaconda3/envs/huggingface/lib/python3.6/site-packages/tensorboard/compat/tensorflow_stub/dtypes.py:543: FutureWarning: Passing (type, 1) or '1type' as a synonym of type is deprecated; in a future version of numpy, it will be understood as (type, (1,)) / '(1,)type'.
  _np_qint16 = np.dtype([("qint16", np.int16, 1)])
/home/<user>/anaconda3/envs/huggingface/lib/python3.6/site-packages/tensorboard/compat/tensorflow_stub/dtypes.py:544: FutureWarning: Passing (type, 1) or '1type' as a synonym of type is deprecated; in a future version of numpy, it will be understood as (type, (1,)) / '(1,)type'.
  _np_quint16 = np.dtype([("quint16", np.uint16, 1)])
/home/<user>/anaconda3/envs/huggingface/lib/python3.6/site-packages/tensorboard/compat/tensorflow_stub/dtypes.py:545: FutureWarning: Passing (type, 1) or '1type' as a synonym of type is deprecated; in a future version of numpy, it will be understood as (type, (1,)) / '(1,)type'.
  _np_qint32 = np.dtype([("qint32", np.int32, 1)])
/home/<user>/anaconda3/envs/huggingface/lib/python3.6/site-packages/tensorboard/compat/tensorflow_stub/dtypes.py:550: FutureWarning: Passing (type, 1) or '1type' as a synonym of type is deprecated; in a future version of numpy, it will be understood as (type, (1,)) / '(1,)type'.
  np_resource = np.dtype([("resource", np.ubyte, 1)])
>>> from transformers import PreTrainedEncoderDecoder
>>> from transformers import BertTokenizer
>>> tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
>>> model = PreTrainedEncoderDecoder.from_pretrained('bert-base-uncased', 'bert-base-uncased')
>>> text='Hi How are you'
>>> import torch
>>> input_ids = torch.tensor(tokenizer.encode(text)).unsqueeze(0)
>>> input_ids
tensor([[ 101, 7632, 2129, 2024, 2017,  102]])
>>> output = model(input_ids) # YOUR PROBLEM IS HERE
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/<user>/anaconda3/envs/huggingface/lib/python3.6/site-packages/torch/nn/modules/module.py", line 541, in __call__
    result = self.forward(*input, **kwargs)
TypeError: forward() missing 1 required positional argument: 'decoder_input_ids'
>>> output = model(input_ids, input_ids) # SOLUTION TO YOUR PROBLEM
>>> output
(tensor([[[ -6.3390,  -6.3664,  -6.4600,  ...,  -5.5354,  -4.1787,  -5.8384],
         [ -6.3550,  -6.3077,  -6.4661,  ...,  -5.3516,  -4.1338,  -4.0742],
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         [-12.8752, -12.3775, -12.2770,  ..., -10.0880, -10.7659,  -9.0092]]],
       grad_fn=<AddBackward0>), tensor([[[ 0.0929, -0.0264, -0.1224,  ..., -0.2106,  0.1739,  0.1725],
         [ 0.4074, -0.0593,  0.5523,  ..., -0.6791,  0.6556, -0.2946],
         [-0.2116, -0.6859, -0.4628,  ...,  0.1528,  0.5977, -0.9102],
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         [-0.0757, -1.3394,  0.1816,  ...,  0.0746,  0.4032, -0.7080],
         [ 0.5989, -0.2841, -0.3490,  ...,  0.3042, -0.4368, -0.2097]]],
       grad_fn=<NativeLayerNormBackward>), tensor([[-9.3097e-01, -3.3807e-01, -6.2162e-01,  8.4082e-01,  4.4154e-01,
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         -9.7611e-01, -6.6894e-01,  3.7154e-01, -9.7862e-01,  9.9949e-01,
         -5.5391e-01, -2.0926e-01, -3.9404e-01, -2.3863e-02,  6.3624e-01,
         -1.0563e-01, -9.8927e-01, -1.4047e-01,  1.2247e-01,  9.7469e-01,
          2.6847e-01, -6.0451e-01, -9.5354e-01,  4.5191e-01,  6.6822e-01,
         -7.2218e-01, -9.6438e-01,  9.7538e-01, -9.9165e-01,  5.6641e-01,
          1.0000e+00,  2.2837e-01, -2.8539e-01,  1.6956e-01, -4.6714e-01,
          2.5561e-01, -2.6744e-01,  7.4301e-01, -9.7890e-01, -2.7469e-01,
         -1.4162e-01,  2.7886e-01, -7.0853e-02, -5.8891e-02,  8.2879e-01,
          1.9968e-01, -5.4085e-01, -6.8158e-01,  3.7584e-02,  3.5805e-01,
          8.9092e-01, -1.7879e-01, -8.1491e-02,  5.0655e-02, -7.9140e-02,
         -9.5114e-01, -1.4923e-01, -3.5370e-01, -9.9994e-01,  7.4321e-01,
         -1.0000e+00,  2.1850e-01, -2.5182e-01, -2.2171e-01,  8.7817e-01,
          2.9648e-01,  3.4926e-01, -8.2534e-01, -3.8831e-01,  7.6622e-01,
          8.0938e-01, -2.1051e-01, -3.0882e-01, -7.6183e-01,  2.2523e-01,
         -1.4952e-02,  1.5150e-01, -2.1056e-01,  7.3482e-01, -1.5207e-01,
          1.0000e+00,  1.0631e-01, -7.7462e-01, -9.8438e-01,  1.6242e-01,
         -1.6337e-01,  1.0000e+00, -9.4196e-01, -9.7149e-01,  3.9827e-01,
         -7.2371e-01, -8.6582e-01,  3.0937e-01, -6.4325e-02, -8.1062e-01,
         -8.8436e-01,  9.8219e-01,  9.3543e-01, -5.6058e-01,  4.5004e-01,
         -3.2933e-01, -5.5851e-01, -6.9835e-02,  6.0196e-01,  9.9111e-01,
          4.1170e-01,  9.1721e-01,  5.9978e-01, -9.4103e-02,  9.7966e-01,
          1.5322e-01,  5.3662e-01,  4.2338e-02,  1.0000e+00,  2.8920e-01,
         -9.3933e-01,  2.4383e-01, -9.8948e-01, -1.5036e-01, -9.7242e-01,
          2.8053e-01,  1.1691e-01,  9.0178e-01, -2.1055e-01,  9.7547e-01,
         -5.0734e-01, -8.5119e-03, -5.2189e-01,  1.1963e-01,  4.0313e-01,
         -9.4529e-01, -9.8752e-01, -9.8975e-01,  4.5711e-01, -4.0753e-01,
          5.8175e-02,  1.1543e-01,  8.6051e-02,  3.6199e-01,  4.3131e-01,
         -1.0000e+00,  9.5818e-01,  4.0499e-01,  6.9443e-01,  9.7521e-01,
          6.7153e-01,  4.3386e-01,  2.2481e-01, -9.9118e-01, -9.9126e-01,
         -3.1248e-01, -1.4604e-01,  7.9951e-01,  6.1145e-01,  9.2726e-01,
          4.0171e-01, -3.9375e-01, -2.0938e-01, -3.2651e-02, -4.1723e-01,
         -9.9582e-01,  4.5682e-01, -9.4401e-02, -9.8150e-01,  9.6766e-01,
         -5.5518e-01, -8.0481e-02,  4.4743e-01, -6.0429e-01,  9.7261e-01,
          8.6633e-01,  3.7309e-01,  9.4917e-04,  4.6426e-01,  9.1590e-01,
          9.6965e-01,  9.8799e-01, -4.6592e-01,  8.7146e-01, -3.1116e-01,
          5.1496e-01,  6.7961e-01, -9.5609e-01,  1.3302e-03,  3.6581e-01,
         -2.3789e-01,  2.6341e-01, -1.2874e-01, -9.8464e-01,  4.8621e-01,
         -1.8921e-01,  6.1015e-01, -4.3986e-01,  2.1561e-01, -3.7115e-01,
         -1.5832e-02, -6.9704e-01, -7.3403e-01,  5.7310e-01,  5.0895e-01,
          9.4111e-01,  6.9365e-01,  6.9171e-02, -7.3277e-01, -1.1294e-01,
         -4.0168e-01, -9.2587e-01,  9.6638e-01,  2.2207e-02,  1.5029e-01,
          2.8954e-01, -8.5994e-02,  7.4631e-01, -1.5933e-01, -3.5710e-01,
         -1.6201e-01, -7.1149e-01,  9.0602e-01, -4.2873e-01, -4.6653e-01,
         -5.4765e-01,  7.4640e-01,  2.3966e-01,  9.9982e-01, -4.6795e-01,
         -6.4802e-01, -4.1201e-01, -3.4984e-01,  3.5475e-01, -5.4668e-01,
         -1.0000e+00,  3.6903e-01, -1.7324e-01,  4.3267e-01, -4.7206e-01,
          6.3586e-01, -5.2151e-01, -9.9077e-01, -1.6597e-01,  2.6735e-01,
          4.5069e-01, -4.3034e-01, -5.6321e-01,  5.7792e-01,  8.8123e-02,
          9.4964e-01,  9.2798e-01, -3.3326e-01,  5.1963e-01,  6.0865e-01,
         -4.4019e-01, -6.8129e-01,  9.3489e-01]], grad_fn=<TanhBackward>))
>>> len(output)
3
>>> output[0]
tensor([[[ -6.3390,  -6.3664,  -6.4600,  ...,  -5.5354,  -4.1787,  -5.8384],
         [ -6.3550,  -6.3077,  -6.4661,  ...,  -5.3516,  -4.1338,  -4.0742],
         [ -6.7090,  -6.6050,  -6.6682,  ...,  -5.9591,  -4.7142,  -3.8219],
         [ -7.7608,  -7.5956,  -7.6634,  ...,  -6.8113,  -5.7777,  -4.1638],
         [ -8.6462,  -8.5767,  -8.6366,  ...,  -7.9503,  -6.5382,  -5.0959],
         [-12.8752, -12.3775, -12.2770,  ..., -10.0880, -10.7659,  -9.0092]]],
       grad_fn=<AddBackward0>)
>>> output[0].shape
torch.Size([1, 6, 30522])
>>> output[1]
tensor([[[ 0.0929, -0.0264, -0.1224,  ..., -0.2106,  0.1739,  0.1725],
         [ 0.4074, -0.0593,  0.5523,  ..., -0.6791,  0.6556, -0.2946],
         [-0.2116, -0.6859, -0.4628,  ...,  0.1528,  0.5977, -0.9102],
         [ 0.3992, -1.3208, -0.0801,  ..., -0.3213,  0.2557, -0.5780],
         [-0.0757, -1.3394,  0.1816,  ...,  0.0746,  0.4032, -0.7080],
         [ 0.5989, -0.2841, -0.3490,  ...,  0.3042, -0.4368, -0.2097]]],
       grad_fn=<NativeLayerNormBackward>)
>>> output[1].shape
torch.Size([1, 6, 768])
>>> output[2]
tensor([[-9.3097e-01, -3.3807e-01, -6.2162e-01,  8.4082e-01,  4.4154e-01,
         -1.5889e-01,  9.3273e-01,  2.2240e-01, -4.3249e-01, -9.9998e-01,
         -2.7810e-01,  8.9449e-01,  9.8638e-01,  6.4763e-02,  9.6649e-01,
         -7.7835e-01, -4.4046e-01, -5.9515e-01,  2.7585e-01, -7.4638e-01,
          7.4700e-01,  9.9983e-01,  4.4468e-01,  2.8673e-01,  3.6586e-01,
          9.7642e-01, -8.4343e-01,  9.6599e-01,  9.7235e-01,  7.2667e-01,
         -7.5785e-01,  9.2892e-02, -9.9089e-01, -1.7004e-01, -6.8200e-01,
         -9.9283e-01,  2.6244e-01, -7.9871e-01,  2.3397e-02,  4.6413e-02,
         -9.3371e-01,  2.7699e-01,  9.9995e-01, -3.2671e-01,  2.1108e-01,
         -2.0636e-01, -1.0000e+00,  1.9622e-01, -9.3330e-01,  6.8736e-01,
          6.4731e-01,  5.3773e-01,  9.2759e-02,  4.1069e-01,  4.0360e-01,
          1.9002e-01, -1.7049e-01,  7.5259e-03, -2.0453e-01, -5.7574e-01,
         -5.3062e-01,  3.9367e-01, -7.0627e-01, -9.2865e-01,  6.8820e-01,
          3.2698e-01, -3.3506e-02, -1.2323e-01, -1.5304e-01, -1.8077e-01,
          9.3398e-01,  2.6375e-01,  3.7505e-01, -8.9548e-01,  1.1777e-01,
          2.2054e-01, -6.3351e-01,  1.0000e+00, -6.9228e-01, -9.8653e-01,
          6.9799e-01,  4.0303e-01,  5.2453e-01,  2.3217e-01, -1.2151e-01,
         -1.0000e+00,  5.6760e-01,  2.9295e-02, -9.9318e-01,  8.3171e-02,
          5.2939e-01, -2.3176e-01, -1.5694e-01,  4.9278e-01, -4.2614e-01,
         -3.8079e-01, -2.6060e-01, -6.9055e-01, -1.7180e-01, -1.9810e-01,
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          4.0373e-01, -2.8194e-01,  4.5104e-01, -9.7876e-01,  6.1044e-01,
         -2.3526e-01, -9.9035e-01, -5.1350e-01, -9.9280e-01,  6.8329e-01,
         -2.1623e-01, -1.4641e-01,  9.8273e-01,  3.7345e-01,  4.8171e-01,
         -5.6467e-03, -7.3005e-01, -1.0000e+00, -7.2252e-01, -5.1978e-01,
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          2.3627e-02, -4.1555e-01,  1.9872e-01, -4.0593e-01,  6.5180e-01,
          6.1598e-01, -6.8750e-01,  7.9808e-02, -2.0437e-01,  3.4504e-01,
         -6.7176e-01, -1.3692e-01, -2.7750e-01, -9.6740e-01, -3.6698e-01,
          9.6934e-01, -2.5050e-01, -6.9297e-01,  4.8327e-01, -1.4613e-01,
         -5.1224e-01,  8.8387e-01,  6.9173e-01,  3.8395e-01, -1.7536e-01,
          3.8873e-01, -4.3011e-03,  6.1876e-01, -8.9292e-01,  3.4243e-02,
          4.5193e-01, -2.4782e-01, -4.7402e-01, -9.8375e-01, -3.1763e-01,
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         -1.8376e-01,  6.8750e-01, -9.7083e-01,  9.8624e-01, -2.0573e-01,
          2.0418e-01,  4.1400e-01,  1.9102e-01, -9.1718e-01, -3.5273e-01,
          8.9628e-01, -5.6812e-01, -8.9552e-01, -3.5567e-02, -4.9052e-01,
         -4.3559e-01, -6.2323e-01,  5.6863e-01, -2.6201e-01, -3.1324e-01,
         -1.2852e-02,  9.4585e-01,  9.8664e-01,  8.3363e-01, -2.4392e-01,
          7.3786e-01, -9.4466e-01, -5.2720e-01, -1.6349e-02,  2.4207e-01,
          3.6905e-02,  9.9638e-01, -5.8095e-01, -7.2046e-02, -9.4418e-01,
         -9.8921e-01, -1.0289e-01, -9.3301e-01, -5.3531e-02, -6.8719e-01,
          5.3295e-01,  1.6390e-01,  2.3460e-01,  4.3260e-01, -9.9501e-01,
         -7.7318e-01,  2.6342e-01, -3.6949e-01,  4.0245e-01, -1.6657e-01,
          4.5766e-01,  7.4537e-01, -5.8549e-01,  8.4632e-01,  9.3526e-01,
         -6.4963e-01, -7.8264e-01,  8.5868e-01, -2.9683e-01,  9.0246e-01,
         -6.5124e-01,  9.8896e-01,  8.6732e-01,  8.7014e-01, -9.5627e-01,
         -4.1195e-01, -9.1043e-01, -4.5438e-01,  5.7729e-02, -3.6862e-01,
          5.7032e-01,  5.5757e-01,  3.0482e-01,  7.0850e-01, -6.6279e-01,
          9.9909e-01, -5.0139e-01, -9.7001e-01, -2.2370e-01, -1.8440e-02,
         -9.9107e-01,  7.2208e-01,  2.4379e-01,  6.9083e-02, -3.2313e-01,
         -7.3217e-01, -9.7295e-01,  9.2268e-01,  5.0675e-02,  9.9215e-01,
         -8.0247e-02, -9.5682e-01, -4.1637e-01, -9.4549e-01, -2.9790e-01,
         -1.5625e-01,  2.4707e-01, -1.8468e-01, -9.7276e-01,  4.7428e-01,
          5.6760e-01,  5.5919e-01, -1.9418e-01,  9.9932e-01,  1.0000e+00,
          9.7844e-01,  9.3669e-01,  9.5284e-01, -9.9929e-01, -4.9083e-01,
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          4.1425e-01, -1.0000e+00, -2.7896e-03,  7.0756e-02, -9.4186e-01,
          2.7960e-01,  9.8389e-01,  9.9658e-01, -1.0000e+00,  8.8289e-01,
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          4.2505e-01,  3.0766e-01, -3.2042e-01,  2.7469e-01, -7.8253e-01,
         -8.8309e-01, -1.5604e-01, -3.6222e-01,  9.9091e-01,  3.0116e-02,
         -7.8697e-01, -9.4496e-01,  2.2050e-01, -8.4521e-02, -4.8378e-01,
         -9.7952e-01, -1.3446e-01,  4.2209e-01,  7.8760e-01,  6.4992e-02,
          2.0492e-01, -7.8143e-01,  2.2120e-01, -5.0228e-01,  3.7149e-01,
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         -2.8122e-01, -9.6984e-01,  9.8036e-01, -3.5220e-01,  7.4903e-01,
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          6.9319e-01,  5.5441e-01, -1.1380e-02,  8.6958e-01, -1.2449e-01,
         -2.8602e-01,  1.8517e-01,  9.2221e-02,  9.6773e-01, -4.5911e-01,
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         -5.5391e-01, -2.0926e-01, -3.9404e-01, -2.3863e-02,  6.3624e-01,
         -1.0563e-01, -9.8927e-01, -1.4047e-01,  1.2247e-01,  9.7469e-01,
          2.6847e-01, -6.0451e-01, -9.5354e-01,  4.5191e-01,  6.6822e-01,
         -7.2218e-01, -9.6438e-01,  9.7538e-01, -9.9165e-01,  5.6641e-01,
          1.0000e+00,  2.2837e-01, -2.8539e-01,  1.6956e-01, -4.6714e-01,
          2.5561e-01, -2.6744e-01,  7.4301e-01, -9.7890e-01, -2.7469e-01,
         -1.4162e-01,  2.7886e-01, -7.0853e-02, -5.8891e-02,  8.2879e-01,
          1.9968e-01, -5.4085e-01, -6.8158e-01,  3.7584e-02,  3.5805e-01,
          8.9092e-01, -1.7879e-01, -8.1491e-02,  5.0655e-02, -7.9140e-02,
         -9.5114e-01, -1.4923e-01, -3.5370e-01, -9.9994e-01,  7.4321e-01,
         -1.0000e+00,  2.1850e-01, -2.5182e-01, -2.2171e-01,  8.7817e-01,
          2.9648e-01,  3.4926e-01, -8.2534e-01, -3.8831e-01,  7.6622e-01,
          8.0938e-01, -2.1051e-01, -3.0882e-01, -7.6183e-01,  2.2523e-01,
         -1.4952e-02,  1.5150e-01, -2.1056e-01,  7.3482e-01, -1.5207e-01,
          1.0000e+00,  1.0631e-01, -7.7462e-01, -9.8438e-01,  1.6242e-01,
         -1.6337e-01,  1.0000e+00, -9.4196e-01, -9.7149e-01,  3.9827e-01,
         -7.2371e-01, -8.6582e-01,  3.0937e-01, -6.4325e-02, -8.1062e-01,
         -8.8436e-01,  9.8219e-01,  9.3543e-01, -5.6058e-01,  4.5004e-01,
         -3.2933e-01, -5.5851e-01, -6.9835e-02,  6.0196e-01,  9.9111e-01,
          4.1170e-01,  9.1721e-01,  5.9978e-01, -9.4103e-02,  9.7966e-01,
          1.5322e-01,  5.3662e-01,  4.2338e-02,  1.0000e+00,  2.8920e-01,
         -9.3933e-01,  2.4383e-01, -9.8948e-01, -1.5036e-01, -9.7242e-01,
          2.8053e-01,  1.1691e-01,  9.0178e-01, -2.1055e-01,  9.7547e-01,
         -5.0734e-01, -8.5119e-03, -5.2189e-01,  1.1963e-01,  4.0313e-01,
         -9.4529e-01, -9.8752e-01, -9.8975e-01,  4.5711e-01, -4.0753e-01,
          5.8175e-02,  1.1543e-01,  8.6051e-02,  3.6199e-01,  4.3131e-01,
         -1.0000e+00,  9.5818e-01,  4.0499e-01,  6.9443e-01,  9.7521e-01,
          6.7153e-01,  4.3386e-01,  2.2481e-01, -9.9118e-01, -9.9126e-01,
         -3.1248e-01, -1.4604e-01,  7.9951e-01,  6.1145e-01,  9.2726e-01,
          4.0171e-01, -3.9375e-01, -2.0938e-01, -3.2651e-02, -4.1723e-01,
         -9.9582e-01,  4.5682e-01, -9.4401e-02, -9.8150e-01,  9.6766e-01,
         -5.5518e-01, -8.0481e-02,  4.4743e-01, -6.0429e-01,  9.7261e-01,
          8.6633e-01,  3.7309e-01,  9.4917e-04,  4.6426e-01,  9.1590e-01,
          9.6965e-01,  9.8799e-01, -4.6592e-01,  8.7146e-01, -3.1116e-01,
          5.1496e-01,  6.7961e-01, -9.5609e-01,  1.3302e-03,  3.6581e-01,
         -2.3789e-01,  2.6341e-01, -1.2874e-01, -9.8464e-01,  4.8621e-01,
         -1.8921e-01,  6.1015e-01, -4.3986e-01,  2.1561e-01, -3.7115e-01,
         -1.5832e-02, -6.9704e-01, -7.3403e-01,  5.7310e-01,  5.0895e-01,
          9.4111e-01,  6.9365e-01,  6.9171e-02, -7.3277e-01, -1.1294e-01,
         -4.0168e-01, -9.2587e-01,  9.6638e-01,  2.2207e-02,  1.5029e-01,
          2.8954e-01, -8.5994e-02,  7.4631e-01, -1.5933e-01, -3.5710e-01,
         -1.6201e-01, -7.1149e-01,  9.0602e-01, -4.2873e-01, -4.6653e-01,
         -5.4765e-01,  7.4640e-01,  2.3966e-01,  9.9982e-01, -4.6795e-01,
         -6.4802e-01, -4.1201e-01, -3.4984e-01,  3.5475e-01, -5.4668e-01,
         -1.0000e+00,  3.6903e-01, -1.7324e-01,  4.3267e-01, -4.7206e-01,
          6.3586e-01, -5.2151e-01, -9.9077e-01, -1.6597e-01,  2.6735e-01,
          4.5069e-01, -4.3034e-01, -5.6321e-01,  5.7792e-01,  8.8123e-02,
          9.4964e-01,  9.2798e-01, -3.3326e-01,  5.1963e-01,  6.0865e-01,
         -4.4019e-01, -6.8129e-01,  9.3489e-01]], grad_fn=<TanhBackward>)
>>> output[2].shape
torch.Size([1, 768])
>>>
```

> @TheEdoardo93 it doesn't matter whether it is GPT2 or bert. Both has the same error :
> I'm trying to play with GPT2 that's why I pasted my own code.
> 
> Using BERT as an Encoder and Decoder
> 
> ```
> >>> model = PreTrainedEncoderDecoder.from_pretrained('bert-base-uncased','bert-base-uncased')
> >>> ouput = model(torch.tensor( encoder_input_ids).unsqueeze(0))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "/home/guest_1/.local/lib/python3.6/site-packages/torch/nn/modules/module.py", line 541, in __call__
>     result = self.forward(*input, **kwargs)
> TypeError: forward() missing 1 required positional argument: 'decoder_input_ids'
> ```

I got the same issue

Do you try to follow my suggestion reported above? In my environment, it **works as expected**.
My environment details:
- **OS**: Ubuntu 16.04
- **Python**: 3.6.9
- **Transformers**: 2.2.2 (installed with `pip install transformers`)
- **PyTorch**: 1.3.1
- **TensorFlow**: 2.0

If not, can you post your **environment** and a list of **steps** to reproduce the bug?

> I got the same issue

>  Said so, I've tested your code and I've revealed how to working with PreTrainedEncoderDecoder correctly without bugs. You can see my code below.

@TheEdoardo93 that doesn't make sense you giving your encoders input as a decoder's input. 



> > Said so, I've tested your code and I've revealed how to working with PreTrainedEncoderDecoder correctly without bugs. You can see my code below.
> 
> @TheEdoardo93 that doesn't make sense your giving your encoders input as a decoder's input.
> Never-mind, I know what's the issue is so closing it.

Sorry, it was my mistake. Can you share with us what was the problem and how to solve it?

@anandhperumal Could you please share how you solved the issue? did you path a ```<BOS>``` to decoder input?
Appreciate that

@anandhperumal can you let us know how you fixed the issue?

You can pass a start token to the decoder, just like a Seq2Seq Arch.