Sagemaker test
# What does this PR do?

This PR creates tests for `SageMaker`. This PR creates tests for the `Pytorch` and `tensorflow` DLC. I added a documentation `README.md` which explains when the Tests need to be run and how. Currently, not all tests are leveraging our `examples/` due to limitations like `SageMakerTrainer` not being integrated into `Trainer` and missing implementation for `keras` for SageMaker specific libraries for data/model parallel. In the near future all scripts in `tests/sagemaker/scripts` are going to be removed and copy the scripts from `examples/` before executing the tests. 

## Current Tests

| ID                                  | description                                                       | plattform                   | #GPUS | collected & evaluated metrics            |
|-------------------------------------|-------------------------------------------------------------------|-----------------------------|-------|------------------------------------------|
| pytorch-transfromers-test-single    | test bert finetuning using BERT fromtransformerlib+PT             | SageMaker createTrainingJob | 1     | train_runtime, eval_accuracy & eval_loss |
| pytorch-transfromers-test-2-ddp     | test bert finetuning using BERT from transformer lib+ PT DPP      | SageMaker createTrainingJob | 16    | train_runtime, eval_accuracy & eval_loss |
| pytorch-transfromers-test-2-smd     | test bert finetuning using BERT from transformer lib+ PT SM DDP   | SageMaker createTrainingJob | 16    | train_runtime, eval_accuracy & eval_loss |
| pytorch-transfromers-test-1-smp     | test roberta finetuning using BERT from transformer lib+ PT SM MP | SageMaker createTrainingJob | 8     | train_runtime, eval_accuracy & eval_loss |
| tensorflow-transfromers-test-single | Test bert finetuning using BERT from transformer lib+TF           | SageMaker createTrainingJob | 1     | train_runtime, eval_accuracy & eval_loss |
| tensorflow-transfromers-test-2-smd  | test bert finetuning using BERT from transformer lib+ TF SM DDP   | SageMaker createTrainingJob | 16    | train_runtime, eval_accuracy & eval_loss |
> LGTM! Should the images be added before merge?

Are you referring to container images? they will be added after the release of `transformers`. If you are referring to the `TODO: Add a screenshot of PR + Text template to make it easy to open.` nope I would add them as soon as we went through the process. To make screenshots while doing it. 