Optionally return acq_values (#3209)

Summary:
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## Motivation

The idea is to optionally returns the acq_values only if needed, else returns None. Also computes them only when necessary to avoid useless forward pass of the acqf. See #3168 for more background.

### Have you read the [Contributing Guidelines on pull requests](https://github.com/meta-pytorch/botorch/blob/main/CONTRIBUTING.md#pull-requests)?

Yes I did.

Pull Request resolved: #3209

Test Plan:
I have updated the API for the `optimize_acqf` and the different flavors of it. I added some test to try out if setting the flag `return_acq_values=False` does indeed return `None` and that by default it returns the `acq_values`.

## Related PRs

No related PRs, only this issue #3168

Reviewed By: hvarfner

Differential Revision: D96154880

Pulled By: Balandat

fbshipit-source-id: 0956e1a419e680b81874c759f98c12b2844466fc

Author: {atinary-lbrey}