reduce duplicate computation in poisson, gamma, and tweedie objectives

[lorentzenchr]

This PR introduces temporaries to save some compuations of std::exp in the regression objectives, namely Poisson, Gamma and Tweedie.

[ghost]

All CLA requirements met.

[jameslamb]

Thanks for your interest in LightGBM!

@lorentzenchr can you please edit the description of this PR to include an explanation of why you'd like to make these changes?

Is the goal mainly to avoid duplicated computation, to speed up calculation of these objectives?

UPD: never mind, I see what you meant now by "save computation". Thank you!

Let me know if you need help understanding how to fix the failing CI jobs.

[lorentzenchr]

CI is green now.

[StrikerRUS]

Thanks a lot for your contribution!

[jameslamb]

very nice, thanks for your help!

[lorentzenchr]

@jameslamb @StrikerRUS The following was brought to my attention: Should this PR be listed in the release notes of https://github.com/microsoft/LightGBM/releases/tag/v4.0.0?

[jameslamb]

@lorentzenchr it is already.

Are you seeing something different?

[lorentzenchr]

@jameslamb I'm so sorry, my browser tricked me somehow.

[jameslamb]

No problem, it's a VERY long page of release notes for this particular release 😂 .

Thanks as always for your contributions here!

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.