code: def testZeroLam(self):
      lam = 0.0
      poisson = poisson_lib.Poisson(rate=lam, validate_args=True)
      self.assertAllClose(lam, self.evaluate(poisson.rate))
      self.assertAllClose(0.0, poisson.prob(3))
      self.assertAllClose(1.0, poisson.prob(0))
      self.assertAllClose(0.0, poisson.log_prob(0))
  
imports: import numpy
  from scipy import stats
  import tensorflow.compat.v2
  from tensorflow_probability.python.distributions import poisson
  from tensorflow_probability.python.distributions.internal import statistical_testing
  from tensorflow_probability.python.internal import dtype_util
  from tensorflow_probability.python.internal import implementation_selection
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.math import gradient
label_desc: Use image node_id 4 for calling the PoissonTest obj's underlying member method code with example usage: obj.testZeroLam() without return types