np.stack(a, axis=0)

>>> import numpy as np
>>> a = np.empty(3, dtype=object)
>>> a[0] = np.array([[1], [2]])
>>> a[1] = np.array([[3], [4]])
>>> a[2] = np.array([[5], [6]])
>>> a
array([array([[1],
       [2]]), array([[3],
       [4]]),
       array([[5],
       [6]])], dtype=object)
>>> a.shape
(3,)
>>> a[0].shape
(2, 1)
>>> np.stack(a, axis=0)
array([[[1],
        [2]],

       [[3],
        [4]],

       [[5],
        [6]]])
>>> np.stack(a, axis=0).shape
(3, 2, 1)