[testing] slow tests should be marked as slow
As discussed in https://github.com/huggingface/transformers/issues/7885 this is an effort to make the test suite manageable execution time-wise, as the number of tests is growing and it takes much longer to complete the tests on CI.

* [x] document and set a standard for when a test needs to be marked as `@slow`
* [x] Marks slow the following tests:
- tests/test_tokenization_auto.py::AutoTokenizerTest::test_tokenizer_from_pretrained (23s)
- tests/test_pipelines.py::MonoColumnInputTestCase::test_torch_translation (17s)
- tests/test_modeling_marian.py::TestMarian_en_ROMANCE::test_pipeline (35s)

Fixes: https://github.com/huggingface/transformers/issues/7885

 @LysandreJik, @sgugger, @sshleifer 

> For common tests to be marked slow, the slowest iteration of that common test must be > 15s.

This one probably needs to be more specific, if for a normal test we decide a threshold of 5s, and you get a common test runs like: 14s, 13, 13, 12s - this is still too slow. perhaps an average of all commont tests for this test should be calculated?

e.g. this one:

```
$ grep test_model_outputs_equivalence stats.txt | perl -ne 's|^(.*?)s.|$x+=$1|e; END {print int $x}'
230
$ grep test_model_outputs_equivalence stats.txt | wc -l
46
$ perl -le 'print 230/46'
5
```
How interesting that it hits 5sec exactly, but it's on my machine so need to re-eval on CI.

Here is a one liner to do all 3 lines at once: (`$.` contains the same data as `wc -l` - line counter in perl)
```
$ grep test_model_outputs_equivalence stats.txt | perl -ne 's|^(.*?)s.|$x+=$1|e; END {print int $x/$.}'
5
```

The full picture for this common test:
```
20.11s call     tests/test_modeling_tf_mobilebert.py::TFMobileBertModelTest::test_model_outputs_equivalence
16.19s call     tests/test_modeling_tf_longformer.py::TFLongformerModelTest::test_model_outputs_equivalence
13.49s call     tests/test_modeling_gpt2.py::GPT2ModelTest::test_model_outputs_equivalence
9.94s call     tests/test_modeling_bert.py::BertModelTest::test_model_outputs_equivalence
9.56s call     tests/test_modeling_albert.py::AlbertModelTest::test_model_outputs_equivalence
8.81s call     tests/test_modeling_flaubert.py::FlaubertModelTest::test_model_outputs_equivalence
8.29s call     tests/test_modeling_tf_xlm.py::TFXLMModelTest::test_model_outputs_equivalence
7.98s call     tests/test_modeling_mobilebert.py::MobileBertModelTest::test_model_outputs_equivalence
7.87s call     tests/test_modeling_xlnet.py::XLNetModelTest::test_model_outputs_equivalence
6.85s call     tests/test_modeling_tf_xlnet.py::TFXLNetModelTest::test_model_outputs_equivalence
6.81s call     tests/test_modeling_squeezebert.py::SqueezeBertModelTest::test_model_outputs_equivalence
6.30s call     tests/test_modeling_tf_roberta.py::TFRobertaModelTest::test_model_outputs_equivalence
6.25s call     tests/test_modeling_roberta.py::RobertaModelTest::test_model_outputs_equivalence
5.90s call     tests/test_modeling_reformer.py::ReformerLocalAttnModelTest::test_model_outputs_equivalence
5.81s call     tests/test_modeling_electra.py::ElectraModelTest::test_model_outputs_equivalence
5.79s call     tests/test_modeling_distilbert.py::DistilBertModelTest::test_model_outputs_equivalence
5.69s call     tests/test_modeling_xlm.py::XLMModelTest::test_model_outputs_equivalence
5.35s call     tests/test_modeling_tf_t5.py::TFT5ModelTest::test_model_outputs_equivalence
4.64s call     tests/test_modeling_tf_funnel.py::TFFunnelBaseModelTest::test_model_outputs_equivalence
4.34s call     tests/test_modeling_tf_bert.py::TFBertModelTest::test_model_outputs_equivalence
3.79s call     tests/test_modeling_dpr.py::DPRModelTest::test_model_outputs_equivalence
3.71s call     tests/test_modeling_tf_funnel.py::TFFunnelModelTest::test_model_outputs_equivalence
3.61s call     tests/test_modeling_bart.py::BARTModelTest::test_model_outputs_equivalence
3.58s call     tests/test_modeling_openai.py::OpenAIGPTModelTest::test_model_outputs_equivalence
3.57s call     tests/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_model_outputs_equivalence
3.53s call     tests/test_modeling_ctrl.py::CTRLModelTest::test_model_outputs_equivalence
3.40s call     tests/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_model_outputs_equivalence
3.31s call     tests/test_modeling_tf_gpt2.py::TFGPT2ModelTest::test_model_outputs_equivalence
3.19s call     tests/test_modeling_tf_electra.py::TFElectraModelTest::test_model_outputs_equivalence
3.12s call     tests/test_modeling_tf_openai.py::TFOpenAIGPTModelTest::test_model_outputs_equivalence
2.98s call     tests/test_modeling_tf_albert.py::TFAlbertModelTest::test_model_outputs_equivalence
2.93s call     tests/test_modeling_tf_flaubert.py::TFFlaubertModelTest::test_model_outputs_equivalence
2.80s call     tests/test_modeling_tf_distilbert.py::TFDistilBertModelTest::test_model_outputs_equivalence
2.59s call     tests/test_modeling_longformer.py::LongformerModelTest::test_model_outputs_equivalence
2.37s call     tests/test_modeling_transfo_xl.py::TransfoXLModelTest::test_model_outputs_equivalence
2.17s call     tests/test_modeling_funnel.py::FunnelModelTest::test_model_outputs_equivalence
2.13s call     tests/test_modeling_fsmt.py::FSMTModelTest::test_model_outputs_equivalence
2.02s call     tests/test_modeling_bert_generation.py::BertGenerationEncoderTest::test_model_outputs_equivalence
1.94s call     tests/test_modeling_funnel.py::FunnelBaseModelTest::test_model_outputs_equivalence
1.70s call     tests/test_modeling_t5.py::T5ModelTest::test_model_outputs_equivalence
1.60s call     tests/test_modeling_deberta.py::DebertaModelTest::test_model_outputs_equivalence
1.44s call     tests/test_modeling_lxmert.py::LxmertModelTest::test_model_outputs_equivalence
1.22s call     tests/test_modeling_layoutlm.py::LayoutLMModelTest::test_model_outputs_equivalence
1.11s call     tests/test_modeling_reformer.py::ReformerLSHAttnModelTest::test_model_outputs_equivalence
0.86s call     tests/test_modeling_tf_ctrl.py::TFCTRLModelTest::test_model_outputs_equivalence
0.33s call     tests/test_modeling_blenderbot.py::BlenderbotTesterMixin::test_model_outputs_equivalence
```
```<|||||>Please have a look at this recent [report](https://github.com/huggingface/transformers/issues/7885#issuecomment-712287004) and let me know if anything else should be marked as slow.
<|||||>@LysandreJik, I think the 3 of us have had a good go at it. At your convenience please review the resulting prose and if there is something to modify please proceed to change it directly and then merge it. Much appreciated!