Updating notebook Pytorch SCVI (#32)

* Updating pytorch_scvi notebook to fix UMAP plots

* `pytorch_scvi.ipynb`: rm metadata, some warnings

* `pytorch_scvi`: update `nnz` comment

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Co-authored-by: Konstantinos Tsitsimpikos <kostas.t@tiledb.com>
Co-authored-by: Ryan Williams <ryan.williams@tiledb.com>

Author: ktsitsi