fix consistency CrossEntropyLoss in modeling_bart
Other modeling architectures (`modeling_*.py`) use `CrossEntropyLoss` and not `F.cross_entropy`.
Fixed this consistency in `modeling_bart.py`.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6265?src=pr&el=h1) Report
> Merging [#6265](https://codecov.io/gh/huggingface/transformers/pull/6265?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d9149f00d1a4650bafa7e1cd73e10398193c852c&el=desc) will **decrease** coverage by `0.14%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6265/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6265?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6265      +/-   ##
==========================================
- Coverage   78.45%   78.31%   -0.15%     
==========================================
  Files         146      146              
  Lines       26595    26596       +1     
==========================================
- Hits        20866    20829      -37     
- Misses       5729     5767      +38     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6265?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6265/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `95.76% <100.00%> (+<0.01%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6265/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.40% <0.00%> (-34.39%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6265/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `80.30% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6265/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `88.19% <0.00%> (+63.97%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6265?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/6265?src=pr&el=footer). Last update [d9149f0...ad31ee8](https://codecov.io/gh/huggingface/transformers/pull/6265?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
