polish concat error message

python/jittor/contrib.py

@@ -211,21 +211,24 @@ def setitem(x, slices, value):
 jt.Var.__getitem__ = jt.Var.slice_var = getitem
 jt.Var.__setitem__ = setitem
 
-def concat(arr, dim):
+def concat(arr, dim=0):
     '''Concat Operator can concat a list of jt Var at a specfic dimension.
     
     * [in] x:   input var list for concat
 
     * [in] dim: concat which dim
 
-    * [out] out:  concat result
+    * return:  concat result
 
 Example::
 
         jt.concat([jt.array([[1],[2]]), jt.array([[2],[2]])], dim=1)
         # return [[1],[2],[2],[2]]
     '''
-    # TODO: low performance when concat lots of vars
+    if not isinstance(arr, Sequence):
+        raise TypeError("concat arr needs to be a tuple or list")
+    if len(arr) == 0:
+        raise ValueError("need at least one array to concat")
     total_dim = 0
     if dim < 0: dim += len(arr[0].shape)
     for a in arr: