Defer pytorch installation to example notebook to speed up binder launch

[JBGreisman]

PyTorch is a very large package, and it seems likely to be the culprit for slowing down the binder launch (regardless of whether a cached docker image is found; #123). This PR defers installation of pytorch to the relevant example notebook. This should decrease the size of the docker image and speed things up, while still ensuring the "robust loss function" example runs.

[codecov-commenter]

Codecov Report

Merging #125 (a6b38a5) into main (d6a3753) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #125   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          41       41           
  Lines        1571     1571           
=======================================
  Hits         1551     1551           
  Misses         20       20           

Flag	Coverage Δ
unittests	98.72% <ø> (ø)

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