Fix indexing with a boolean nnabla.Variable or NdArray

[TE-StephenTiedemann]

Indexing with a boolean array must produce the index at positions where the index contains True values. This worked with lists and numpy arrays but went wrong for nnabla.NdArray or nnabla.Variable of boolean values:

>>> x = nnabla.Variable.from_numpy_array(numpy.array([11, 22, 33]))
>>> i = nnabla.Variable.from_numpy_array(numpy.array([True, False, True]))
>>> x[i.d].d == x[[True, False, True]].d
array([ True,  True])
>>> x[i.d].d == x[i].d
False

This PR extends the implementation such that x[i].d == x[i.d].d is all true.