Zero gradient for concatenate

[wddabc]

Looks like the computation graph breaks on the concatenation operation.
MWE:

import minpy.numpy as np
from minpy.core import grad

def foo_nocat(x):
    return 3*x 

def foo_cat(x):
    catx = np.concatenate([x, x], axis=1)
    return np.dot(catx, np.array([[1], [2]]))

test_x = np.array([[3]]) 
print grad(foo_nocat)(test_x)  # correct_output 
print grad(foo_cat)(test_x)  # should be the same

[jermainewang]

@ZihengJiang Could you have a look? Also put this in unittest.

[Taco-W]

@ZihengJiang Any follow-up on this?