Add Pytorch Native AMP support in Trainer
Pytorch 1.6 introduces Native AMP support. This eliminates the need to build and install Apex and provides improvements over problems highlighted in [Apex #818](https://github.com/NVIDIA/apex/issues/818) and flexibility. This is the recommended way to use AMP.

With this PR, Trainer will automatically use Pytorch's native AMP if 1.6 version is installed, otherwise, it will use Apex.

This PR will close [#6115](https://github.com/huggingface/transformers/issues/6115). 