Fixes Issue #2781 for gcloud command in getting number of workers

[MrGeislinger]

Description

This simply fixes the gcloud command to find the number of workers in a multihost run. It adds a "tpu-vm" as part of the subcommand in the gcloud command.

FIXES: #2781

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Tests

Running the following no longer stops on the error on L182:

python3 maxtext/tools/orchestration/multihost_runner.py \
  --ZONE=$ZONE \
  --TPU_PREFIX=$QR_NAME \
  --COMMAND="bash maxtext/tools/setup/setup.sh MODE=nightly LIBTPU_GCS_PATH=${LIBTPU_GCS_PATH}"

Checklist

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