:speech_balloon: Show how to convert xarray.DataArray to torch.Tensor

[weiji14]

Now we can load xarray.DataArray objects into torch.Tensor via a DataPipe! Patches #8.

References:

• https://pytorch.org/data/0.4.0/generated/torchdata.datapipes.iter.Mapper.html
• https://wandb.ai/capecape/torchdata/reports/How-the-TorchData-API-Works-a-Tutorial-with-Code--VmlldzoxNjkwNjIz
• https://github.com/tcapelle/torchdata/blob/4a22188f0f23e2d95e988a794ee5aaec197c16c5/01_Camvid_segmentation_with_datapipes.ipynb