[BART] Update encoder and decoder on set_input_embedding
Since  _resize_token_embeddings will create a new embedding layer,
resizing the input embeddings for BART will currently break, as the
model.shared will refer to the new embedding created but the
model.encoder.embed_tokens and the model.decoder.embed_tokens will
still refer to the old embedding created.

We need to re-assign the encoder/decoder or just their weights, I opted for the second option.
Unfortunately can't see how to write a test in the test_resize_tokens_embeddings to capture this without putting a BART-specific if statement there, but this is also related to https://github.com/huggingface/transformers/issues/3378

Run tests:
 733 passed, 319 skipped, 80 warnings in 269.62s (0:04:29) 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3501?src=pr&el=h1) Report
> Merging [#3501](https://codecov.io/gh/huggingface/transformers/pull/3501?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/601ac5b1dc1438f00d09696588f2deb0f045ae3b&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3501/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3501?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3501   +/-   ##
=======================================
  Coverage   77.79%   77.80%           
=======================================
  Files         100      100           
  Lines       17051    17053    +2     
=======================================
+ Hits        13265    13268    +3     
+ Misses       3786     3785    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3501?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3501/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `97.59% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3501/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.94% <0.00%> (+0.13%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3501?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3501?src=pr&el=footer). Last update [601ac5b...5e11181](https://codecov.io/gh/huggingface/transformers/pull/3501?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
