Fix ndarray assignment issue with basic indexing

[reminisce]

The following code would throw exception before this fix because the value was not broadcast to the right shape .

data = mx.nd.ones((2, 2))  # data.shape = (2, 2)
value = mx.nd.array([3])  # value.shape = (1,)
data[:] = value  # the value should be broadcast to shape (2, 2) first and then call copyto(data)