implement digamma function

digamma.diff

diff --git a/tensorflow/python/ops/math_grad.py b/tensorflow/python/ops/math_grad.py
index 9e255c2..58338d1 100644
--- a/tensorflow/python/ops/math_grad.py
+++ b/tensorflow/python/ops/math_grad.py
@@ -266,7 +266,15 @@ def _ErfcGrad(op, grad):
 @ops.RegisterGradient("Lgamma")
 def _LgammaGrad(op, grad):  # pylint: disable=unused-argument
   # TODO(ebrevdo): implement digamma
-  raise NotImplementedError("grad(Lgamma) == Digamma is not implemented")
+
+  x = op.inputs[0]
+  z = x-1
+  n = constant_op.constant( np.linspace(1,100,100).reshape((100,1)), dtype=grad.dtype )
+  euler_gamma = constant_op.constant( np.euler_gamma, dtype=grad.dtype )
+  with ops.control_dependencies([grad.op]):
+    return  grad * (-euler_gamma + math_ops.reduce_sum(z/(n*(n+z)),0))
+
+  #raise NotImplementedError("grad(Lgamma) == Digamma is not implemented")
 
 
 @ops.RegisterGradient("Sigmoid")