fix(imperative/cpu/multithread): fix multithread at imperative

GitOrigin-RevId: 9120d5cb48e1087f38a826d377ca3c6bae984730

imperative/python/megengine/device.py

@@ -50,7 +50,9 @@ _sh = _stream_helper()
 
 
 def _valid_device(inp):
-    if isinstance(inp, str) and re.match("^([cxg]pu|rocm)(\d+|\d+:\d+|x)$", inp):
+    if isinstance(inp, str) and re.match(
+        "^([cxg]pu|rocm|multithread)(\d+|\d+:\d+|x)$", inp
+    ):
         return True
     return False