Add single-cell RNA seq example

[agrawalraj]

We can look at this open-source project to easily load in single-cell RNA seq data in batches in pytorch:

https://chanzuckerberg.github.io/cellxgene-census/notebooks/experimental/pytorch.html

[agrawalraj]

When I tried installing cellxgene-census, the alias dependency requires version 23.1.0 or above. I initially had an earlier version and got the error TypeError: field() got an unexpected keyword argument 'alias'. To use the PyTorch dataloader by importing import cellxgene_census.experimental.ml as census_ml, you need to install torchdata (missing dependency in the cellxgene-census package).

[qinqian]

scvi has a more mature api for batch operation of single cell data with pytorch datasets. please see here.