Assemble tensors from nested lists of blocks

[hekaisheng]

Is your feature request related to a problem? Please describe.
Numpy supports assembling an ndarray from nested lists of blocks in version 1.13.0, we can add the same interface like mt.block to build a block matrix.

In [1]: import mars.tensor as mt                                                                                                                     

In [2]: a = mt.array([1, 2, 3])                                                                                                                      

In [3]: b = mt.array([2, 3, 4])                                                                                                                      

In [4]: c = mt.block([[a], [b]])                                                                                                                     

In [5]: c.execute() 
Out[5]: 
array([[1, 2, 3],
       [2, 3, 4]])

Describe the solution you'd like
The implementation may be similar with concatenate.