[Bart] enable test_torchscript, update test_tie_weights
This sets `test_torchscript=True` for BART and removes unneeded asserts in `test_tie_weights`.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5457?src=pr&el=h1) Report
> Merging [#5457](https://codecov.io/gh/huggingface/transformers/pull/5457?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/306f1a269504b781f886d75105acabf8ae95bd11&el=desc) will **decrease** coverage by `0.28%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5457/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5457?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5457      +/-   ##
==========================================
- Coverage   77.86%   77.57%   -0.29%     
==========================================
  Files         141      141              
  Lines       24608    24608              
==========================================
- Hits        19160    19089      -71     
- Misses       5448     5519      +71     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5457?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5457/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxuZXQucHk=) | `32.43% <0.00%> (-55.86%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5457/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5457/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.18% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5457/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.59% <0.00%> (+0.33%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5457/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.37% <0.00%> (+25.00%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5457?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5457?src=pr&el=footer). Last update [306f1a2...e08f8b7](https://codecov.io/gh/huggingface/transformers/pull/5457?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
