Examples reorg
# What does this PR do?

As discussed internally, this PR reorganizes the `examples` folder to make clean subfolders for PyTorch and TensorFlow. This way, each example can have its own requirements including the proper backend and there is no headache to determine who will be first in the README.

It also splits the seq2seq folder in two: translation and summarization.

Finally it moves the content of `examples/test_data` to `tests/fixtures/tests_samples` which is more adapted.

In passing, it updates references to the examples that were moved.
`tests/deepspeed` and `tests/extended` need to be updated too following the rename. Thank you.

```
pip install fairscale deepspeed
RUN_SLOW=1 pytest tests/deepspeed tests/extended
```<|||||>Which tests are failing for you in `tests/extended`? Everything is passing on my side. For deepspeed I fixed the last path I had forgotten to update but it's impossible for me to run those tests as they all error out since deepspeed is not able to build properly on my setup. Got:

```
                                 !! WARNING !!                                                                                                                                                                                                                                                                                                                                                                                              !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Your compiler (c++) is not compatible with the compiler Pytorch was     
 built with for this platform, which is g++ on linux. Please
use g++ to to compile your extension. Alternatively, you may
compile PyTorch from source using c++, and then you can also use
c++ to compile your extension. 

See https://github.com/pytorch/pytorch/blob/master/CONTRIBUTING.md for help
with compiling PyTorch from source. 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
                                !! WARNING !!         
```
and pretty much every test is a failure.<|||||>All tests now pass (including `fairscale` and `apex`) - one deepspeed test fails but it's unrelated to this PR.

If you have trouble building deepspeed at run time, please consider pre-building it:  https://huggingface.co/transformers/master/main_classes/trainer.html#installation and of course report an Issue to Deepspeed if you have a few minutes to do so.

Thank you for fixing this, @sgugger <|||||>Looks like we have some really old dead references too:
```
docs/source/testing.rst:* :prefix_link:`test_seq2seq_examples_multi_gpu.py <examples/seq2seq/test_seq2seq_examples_multi_gpu.py>` - a
docs/source/testing.rst:* :prefix_link:`test_finetune_trainer.py <examples/seq2seq/test_finetune_trainer.py>` - a normal (non-PL) test
docs/source/testing.rst:    CUDA_VISIBLE_DEVICES="0,1" RUN_SLOW=1 pytest -sv examples/seq2seq/test_finetune_trainer.py \
docs/source/testing.rst:    examples/seq2seq/test_seq2seq_examples_multi_gpu.py
docs/source/testing.rst:            data_dir = self.examples_dir / "seq2seq/test_data/wmt_en_ro"
```<|||||>Yes indeed! Those are for the very old scripts (that's not the only place we have those). I wasn't sure how to replace those so if you could point me to the script you want to use instead, I can adapt. I think the whole paragraph may need a rewrite since it has been a long time.<|||||>> Yes indeed! Those are for the very old scripts (that's not the only place we have those). I wasn't sure how to replace those so if you could point me to the script you want to use instead, I can adapt. I think the whole paragraph may need a rewrite since it has been a long time.

Fixed here https://github.com/huggingface/transformers/pull/11359