Rework pipeline tests
# What does this PR do?

The test fetcher has been pretty good at identifying which tests to run and even if we're not testing everything on each commit anymore, we've mostly avoided bad surprises. 

Except for pipeline tests.

This is because the pipeline tests are structured in a way that makes it hard for the test fetcher to guess it has to run them, as they don't see to rely on anything else than the pipeline.

Also running pipeline tests is annoying as you have to remember to activate a special env variable. It made sense back in the days where we had all tests in one folder, but now that they are nicely structured, this is completely unnecessary.

Thus this PR proposes two things:
1. remove the special marker for pipeline tests and the corresponding env variable. We know they are all in `tests/pipelines`.
2. run all pipeline tests any time there is some code change warranting at least one test, like we do for the examples. They are taking the same time roughly, and since the pipelines are a good integration test, I think it actually makes more sense to test those all the times than the examples.
Note: Flax tests are currently failing because it tries to run the tests inside the pipelines folder, will fix this tomorrow by having all non-pipeline jobs not run any of the tests in the pipelines folder. The `--ignore` flag from pytest does not work for some reason, but the test fetcher can probably fix that somehow ^^.<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>I love this PR !

> Note: Flax tests are currently failing because it tries to run the tests inside the pipelines folder

Can't we make the tests parsable even when having neither PT nor TF ?

Here this:

`from transformers import DetrForSegmentation` seems to be the culprit (in `tests/pipelines/test_pipelines_for_segmentation.py`).

Shouldn't we have dummy models when `torch` is not available ?<|||||>> Can't we make the tests parsable even when having neither PT nor TF ?

I can certainly do that too, but the pipeline tests are isolated to not be run by the other jobs, so they shouldn't even be run.<|||||>> I can certainly do that too, but the pipeline tests are isolated to not be run by the other jobs, so they shouldn't even be run.

I'm thinking about regular user using JAX (or non libraries actually) doing `pytest -sv tests/` . IMO it'd be nice if the command ran instead of crashing.