hide unused loop variables

[jameslamb]

Contributes to #333.

Resolves these issues found by ruff when asking it to run over notebook files:

source/examples/rapids-autoscaling-multi-tenant-kubernetes/notebook.ipynb:cell 62:9:9: B007 Loop control variable `stage` not used within loop body
source/examples/time-series-forecasting-with-hpo/notebook.ipynb:cell 143:19:9: B007 Loop control variable `store_id` not used within loop body
source/examples/time-series-forecasting-with-hpo/notebook.ipynb:cell 143:20:13: B007 Loop control variable `dept_id` not used within loop body
source/examples/time-series-forecasting-with-hpo/notebook.ipynb:cell 143:26:29: B007 Loop control variable `valid_mask` not used within loop body

Notes for Reviewers

This isn't all of the things ruff found, but just trying to do them in small batches to make the reviews easier.

[review-notebook-app[bot]]

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