torch.digamma (pytorch#10967)

Summary:
Fixes pytorch#10307

cc SsnL
Pull Request resolved: pytorch#10967

Differential Revision: D9546748

Pulled By: zou3519

fbshipit-source-id: 764e27b1cc8dd487270b3ffa653b806c86f717dd

docs/source/torch.rst

@@ -169,6 +169,7 @@ Pointwise Ops
 .. autofunction:: cos
 .. autofunction:: cosh
 .. autofunction:: div
+.. autofunction:: digamma 
 .. autofunction:: erf
 .. autofunction:: erfc
 .. autofunction:: erfinv

torch/_torch_docs.py

@@ -1168,6 +1168,26 @@ def parse_kwargs(desc):
              [ 1.0500,  0.7336, -0.3836, -1.1015]]])
 """)
 
+add_docstr(torch.digamma,
+           r"""
+digamma(input) -> Tensor
+
+Computes the logarithmic derivative of the gamma function on `input`.
+
+.. math::
+    \psi(x) = \frac{d}{dx} \ln\left(\Gamma\left(x\right)\right) = \frac{\Gamma'(x)}{\Gamma(x)}
+
+Args:
+    input (Tensor): the tensor to compute the digamma function on
+
+Example::
+
+    >>> a = torch.tensor([1, 0.5])
+    >>> torch.digamma(a)
+    tensor([-0.5772, -1.9635])
+""")
+
+
 add_docstr(torch.dist,
            r"""
 dist(input, other, p=2) -> Tensor