Force old coreml api to fix failing test

Between the previous ci_gpu image and the one in use now, the version of
coreml has changed since it is unpinned (6.3.0 -> 7.0.0). This caused a
test failure in the coreml frontend tests. The problem is that coreml
now uses a new default format for models. This commit forces coreml to
use the old model format so the test passes.

More information can be found here: https://apple.github.io/coremltools/docs-guides/source/convert-learning-models.html

Change-Id: Id2273ccac0ae771f10dda6590e9f4cdd657a5d25

Author: lhutton1

tests/python/frontend/coreml/test_forward.py

@@ -823,7 +823,9 @@ def test_can_build_keras_to_coreml_to_relay():
         kmodel_fn = path.join(tmpdir, "c1mdl.h5")
         model.save(kmodel_fn)
 
-        mdl = cm.convert(kmodel_fn)
+        mdl = cm.convert(
+            kmodel_fn, convert_to="neuralnetwork", minimum_deployment_target=cm.target.macOS11
+        )
         model_file = path.join(tmpdir, "c1.mlmodel")
         mdl.save(model_file)