Fix test guards

Change-Id: Icb8f0fd19645f0c0b2730e8db0d2d2b41a34e2c1

Author: mbaret

tests/python/contrib/test_ethosu/cascader/test_graph.py

@@ -14,6 +14,10 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
+import pytest
+
+pytest.importorskip("ethosu.vela")
+
 from tvm.contrib.ethosu.cascader import (
     StripeConfig,
     Propagator,
@@ -23,8 +27,6 @@
     CascaderGraph,
 )
 
-import pytest
-
 
 def test_tensor():
     shape = [1, 2, 3]