!5602 Fix log prob and survival function of exponential distribution

Merge pull request !5602 from XunDeng/pp_issue_branch

mindspore/nn/probability/distribution/exponential.py

@@ -198,9 +198,9 @@ class Exponential(Distribution):
         return self._entropy(rate) + self._kl_loss(dist, rate_b, rate)
 
 
-    def _prob(self, value, rate=None):
+    def _log_prob(self, value, rate=None):
         r"""
-        pdf of Exponential distribution.
+        log_pdf of Exponential distribution.
 
         Args:
             Args:
@@ -211,15 +211,16 @@ class Exponential(Distribution):
             Value should be greater or equal to zero.
 
         .. math::
-            pdf(x) = rate * \exp(-1 * \lambda * x) if x >= 0 else 0
+            log_pdf(x) = \log(rate) - rate * x if x >= 0 else 0
         """
         value = self._check_value(value, "value")
         value = self.cast(value, self.dtype)
         rate = self._check_param(rate)
-        prob = self.exp(self.log(rate) - rate * value)
+        prob = self.log(rate) - rate * value
         zeros = self.fill(self.dtypeop(prob), self.shape(prob), 0.0)
+        neginf = self.fill(self.dtypeop(prob), self.shape(prob), -np.inf)
         comp = self.less(value, zeros)
-        return self.select(comp, zeros, prob)
+        return self.select(comp, neginf, prob)
 
     def _cdf(self, value, rate=None):
         r"""
@@ -243,6 +244,27 @@ class Exponential(Distribution):
         comp = self.less(value, zeros)
         return self.select(comp, zeros, cdf)
 
+    def _log_survival(self, value, rate=None):
+        r"""
+        log survival_function of Exponential distribution.
+
+        Args:
+            value (Tensor): value to be evaluated.
+            rate (Tensor): rate of the distribution. Default: self.rate.
+
+        Note:
+            Value should be greater or equal to zero.
+
+        .. math::
+            log_survival_function(x) = -1 * \lambda * x if x >= 0 else 0
+        """
+        value = self._check_value(value, 'value')
+        value = self.cast(value, self.dtype)
+        rate = self._check_param(rate)
+        sf = -1. * rate * value
+        zeros = self.fill(self.dtypeop(sf), self.shape(sf), 0.0)
+        comp = self.less(value, zeros)
+        return self.select(comp, zeros, sf)
 
     def _kl_loss(self, dist, rate_b, rate=None):
         """