Speedup torch summarization tests
Speedup torch summarization tests  by using small models that are faster to download and instantiate.
Non slow test speed before change:
![image](https://user-images.githubusercontent.com/6045025/78608104-459d8000-782e-11ea-9a30-a16ea20e3eec.png)
<|||||>There is a tiny TF T5 model now as well via: 
`model = TFAutoModelWithLMHead.from_pretrained("patrickvonplaten/t5-tiny-random")`<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3663?src=pr&el=h1) Report
> Merging [#3663](https://codecov.io/gh/huggingface/transformers/pull/3663?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0a9d09b42a9c7c1ccc00da48486a1188078e8594&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3663/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3663?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3663      +/-   ##
==========================================
- Coverage   78.03%   78.02%   -0.02%     
==========================================
  Files         104      104              
  Lines       17708    17708              
==========================================
- Hits        13819    13817       -2     
- Misses       3889     3891       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3663?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3663/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.96% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3663/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.97% <0.00%> (-0.13%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3663?src=pr&el=footer). Last update [0a9d09b...4751188](https://codecov.io/gh/huggingface/transformers/pull/3663?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM, but FYI for context, I think the reason we were using the real models was that we intended to do integration testing: in `_test_mono_column_pipeline` we only test equality of keys but we could have tested equality (or closeness) of values.<|||||>Makes sense @julien-c .  I'd be happy to add some `@slow` integration tests and try to  fulfill the original intent