Add ciflow/cuda label to weekly PyTorch pin bump PRs (pytorch#23136)

## Summary

The weekly PyTorch pin bump workflow (`weekly-pytorch-pin-bump.yml`, run
by pytorchupdatebot) now adds the `ciflow/cuda` label when it creates
the PR. This makes the CUDA CI (`cuda.yml`, `cuda-windows.yml`) run on
pin bumps, so CUDA breakages from new PyTorch nightlies show up on the
bump PR itself.

## Test plan

Workflow-only change. Verify on the next scheduled run (or a manual
`workflow_dispatch`) that the bump PR has the `ciflow/cuda` label and
that the CUDA workflows are triggered.

Author: Gasoonjia

.github/workflows/weekly-pytorch-pin-bump.yml

@@ -90,4 +90,5 @@ jobs:
           gh pr create \
             --title "Bump PyTorch pin to nightly ${NIGHTLY}" \
             --body "${PR_BODY}" \
-            --label "ci/pytorch-pin-bump"
+            --label "ci/pytorch-pin-bump" \
+            --label "ciflow/cuda"