docs: def validate_stores_secrets_keys(stores_secrets):
          """Validates that we can only authenticate to one S3 and one GCS."""
          stores = set([])
          for store_secret in stores_secrets:
              if store_secret['store'] in stores:
                  raise TensorboardValidation('Received an invalid store configuration.')
              elif store_secret['store'] not in {GCS, S3}:
                  raise TensorboardValidation('Received an unsupported store configuration.')
              stores.add(store_secret['store'])