Renaming dask-pytorch package to dask-pytorch-ddp

To make user experience easier and clearer, we're changing the name of our existing library. dask-pytorch is deprecated and may be used in the future for a more multifaceted package. Since our work is primarily focused on making the Distributed Data Parallel functionality of PyTorch work smoothly with Dask, we chose dask-pytorch-ddp for the new name.