Added posterior_transform to posterior method in ApproximateGPyTorchModel

[SaiAakash]

Motivation

This PR fixes #2530. Adds a new posterior_transform parameter to the posterior method of ApproximateGPyTorchModel.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

I was able to generate candidate points with ExpectedImprovement acquisition function with a SingleTaskVariationalGP that was trained on 2 output columns.

Related PRs

NA

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.98%. Comparing base (6ebfa82) to head (a8f7f5a). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2531 +/- ## ======================================= Coverage 99.98% 99.98% ======================================= Files 193 193 Lines 17006 17010 +4 ======================================= + Hits 17003 17007 +4 Misses 3 3 ```

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[SaiAakash]

@Balandat I have addressed the requested changes.

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@saitcakmak has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

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@Balandat has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@Balandat has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

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@Balandat has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@Balandat merged this pull request in pytorch/botorch@e9ce11fd50fee36db017f0f5938a1418bb598aeb.