implement random method for wishart (#2473)

For potential usage in SMC/ABC where we need random method to generate sample. Still not encourage to use.

Author: Junpeng Lao

pymc3/distributions/multivariate.py

@@ -650,10 +650,16 @@ def __init__(self, nu, V, *args, **kwargs):
         self.p = p = tt.as_tensor_variable(V.shape[0])
         self.V = V = tt.as_tensor_variable(V)
         self.mean = nu * V
-        self.mode = tt.switch(1 * (nu >= p + 1),
+        self.mode = tt.switch(tt.ge(nu, p + 1),
                               (nu - p - 1) * V,
                               np.nan)
 
+    def random(self, point=None, size=None): 
+        nu, V = draw_values([self.nu, self.V], point=point) 
+        size= 1 if size is None else size
+        return generate_samples(stats.wishart.rvs, np.asscalar(nu), V, 
+                                    broadcast_shape=(size,)) 
+
     def logp(self, X):
         nu = self.nu
         p = self.p