update the config.is_decoder=True before initialize the decoder
Currently the `PreTrainedEncoderDecoder`  class fails to initialize the "cross-attention layer" since it updates  `decoder.config.is_decoder = True` after decoder initialization.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2389?src=pr&el=h1) Report
> Merging [#2389](https://codecov.io/gh/huggingface/transformers/pull/2389?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9261c7f771fccfa2a2cb78ae544adef2f6eb402b?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2389/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2389?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2389   +/-   ##
=======================================
  Coverage   73.24%   73.24%           
=======================================
  Files          87       87           
  Lines       15001    15001           
=======================================
  Hits        10988    10988           
  Misses       4013     4013
```



------

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<|||||>But these are not decoders, they're encoders with an additional language modeling head?<|||||>> But these are not decoders, they're encoders with an additional language modeling head?

Oh, thanks to point out my mistake, I should actually modify the `modeling_encoder_decoder.py` file. I accidentally closed this pull request and made a new one #2435 . 