name 'concatenate' is not defined

Hi all,

I am trying to run @vasilismsr prototype NLSYM_Linear. Everything works fine except when I run cate.fit().
Specifically I have the error:

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-19-6ec0fce7800f> in <module>
----> 1 cate.fit(y, T, X, Z)

~/EconML/prototypes/dml_iv/dml_iv.py in fit(self, y, T, X, Z, store_final)
    126         for idx, (train, test) in enumerate(splits):
    127             # Estimate h(Z, X) = E[T | Z, X] in cross-fitting manner
--> 128             proj_t[test] = self.model_T_XZ[idx].fit(hstack([X[train], Z[train]]),
    129                                                     T[train]).predict(hstack([X[test],
    130                                                                               Z[test]]))

~/EconML/prototypes/dml_iv/dml_iv.py in hstack(XS)
     37     """
     38     # Confusingly, this needs to concatenate, not stack (stack returns an array with an extra dimension)
---> 39     return concatenate(XS, 1)
     40 
     41 

NameError: name 'concatenate' is not defined

which I guess deals with utilities.py, i.e. it does not find concatenate which is used in hstack. I checked and concatenate seems to be well defined:

    """
    Join a sequence of arrays along an existing axis.

    Parameters
    ----------
    X1, X2, ... : sequence of array_like
        The arrays must have the same shape, except in the dimension
        corresponding to `axis` (the first, by default).

    axis : int, optional
        The axis along which the arrays will be joined.  Default is 0.

    Returns
    -------
    ndarray or SparseArray
        The concatenated array. It will be sparse if the inputs are.
    """
    def conc(*XS):
        return sp.concatenate(XS, axis=axis) if iscoo(XS[0]) else np.concatenate(XS, axis=axis)
    return _apply(conc, *XS)

Start in prototypes/dml_iv/dml_iv.py at hstack and inspect how concatenate is referenced, then compare that with the concatenate implementation shown from utilities.py. Confirm the missing name is available where hstack runs, and verify that cate.fit(y, T, X, Z) no longer raises this NameError.