!48176 修改argmin接口入参默认值

Merge pull request !48176 from 李良灿/r2.0.0-alpha

mindspore/python/mindspore/ops/function/math_func.py

@@ -436,7 +436,7 @@ def exp2(x):
     return exp2_(tensor_2, x)
 
 
-def argmin(x, axis=-1, keepdims=False):
+def argmin(x, axis=None, keepdims=False):
     """
     Returns the indices of the minimum value of a tensor across the axis.