[Bugfix] Fix target host for vm compiler (apache#4057)

* fix

* tweak

Author: icemelon

python/tvm/relay/backend/vm.py

@@ -178,6 +178,11 @@ def compile(self, mod, target=None, target_host=None):
         """
         target = _update_target(target)
         target_host = None if target_host == "" else target_host
+        if not target_host:
+            for device_type, tgt in target.items():
+                if device_type.value == tvm.nd.cpu(0).device_type:
+                    target_host = tgt
+                    break
         if not target_host:
             target_host = "llvm" if tvm.module.enabled("llvm") else "stackvm"
         target_host = tvm.target.create(target_host)