approxhpvm.py codegen_target is required

hpvm/tools/py-approxhpvm/main.py.in

@@ -146,6 +146,7 @@ def parse_args():
         "-t",
         "--codegen-target",
         type=str,
+        required=True,
         choices=["tensor", "cudnn"],
         help="Backend to use",
     )