[deepspeed tests] transition to new tests dir
as discussed at https://github.com/huggingface/transformers/issues/10076 relocating deepspeed tests a dedicated area and out of the scripts area.

I went right ahead and create a dedicated sub-folder for deepspeed tests.

I no longer can use the libraries from `seq2seq` since I can't do a relative import from the script. 

The only thing is that I will need to update all of my comments/posts to mention that `ds_config.json` has moved.

fairscale will be probably next if this looks good.

Fixes: https://github.com/huggingface/transformers/issues/10076

@sgugger 
> I no longer can use the libraries from seq2seq since I can't do a relative import from the script.

We can add things to the sys path if needed (see the [general text examples](https://github.com/huggingface/transformers/blob/master/examples/test_examples.py)).

Thanks for doing this, it looks good to me!<|||||>Indeed, that's what we have been doing, but having a library named "`utils.py`" and importing it from far away is too ambiguous. So will probably need to rename such libraries, or start moving their functionality into a more central area.