## What changes were proposed in this pull request?
It is reported by Spark users that the deviance calculation for poisson regression does not handle y = 0. Thus, the correct model summary cannot be obtained. The user has confirmed the the issue is in
```
override def deviance(y: Double, mu: Double, weight: Double): Double =
{ 2.0 * weight * (y * math.log(y / mu) - (y - mu)) }
when y = 0.
```
The user also mentioned there are many other places he believe we should check the same thing. However, no other changes are needed, including Gamma distribution.
## How was this patch tested?
Add a comparison with R deviance calculation to the existing unit test.
Author: Teng Peng <josephtengpeng@gmail.com>
Closes#21125 from tengpeng/Spark24024GLM.
293a0f29e3