std(axis) returns an array (??) of n DNDarrays instead of a DNDarray of size n

[ClaudiaComito]

Description

See title

To Reproduce

>>> a = ht.random.randn(4,4)
>>> a
DNDarray([[-1.8112, -1.5437, -0.5754,  0.6996],
          [-1.1083,  0.5049,  1.2382, -0.2294],
          [ 0.6129,  0.4115, -0.0879,  0.5323],
          [-0.5036,  1.1924, -0.6366, -0.5685]], dtype=ht.float32, device=cpu:0, split=None)
>>> ht.std(a, 0, ddof=1)
array([DNDarray(1.0269, dtype=ht.float32, device=cpu:0, split=None),
       DNDarray(1.1760, dtype=ht.float32, device=cpu:0, split=None),
       DNDarray(0.8711, dtype=ht.float32, device=cpu:0, split=None),
       DNDarray(0.6060, dtype=ht.float32, device=cpu:0, split=None)],
      dtype=object)

Expected behavior
Example with axis=1:

>>> ht.std(a, 1)
DNDarray([1.1364, 1.0046, 0.3145, 0.8827], dtype=ht.float32, device=cpu:0, split=None)

Version Info
release/1.0.x and current master