fix dirichlet

Author: NKNaN

python/paddle/distribution/dirichlet.py

@@ -71,7 +71,7 @@ class Dirichlet(exponential_family.ExponentialFamily):
  """
 
  def __init__(self, concentration):
- if concentration.dim() < 1 or math.prod(concentration.shape) < 1:
+ if concentration.dim() < 1 or math.prod(concentration.shape) == 0:
  # 0-dim tensor or 0-sized tensor is invalid
  raise ValueError(
  "`concentration` parameter must be at least one dimensional"