Change defaults MC Samples to 512 (#1535)

Summary:
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## Motivation

I just recognized that in `get_acquisition_function` a default of 500 mc samples is used, but when one initializes an MC acquisition function directly a default of 512 samples is used: https://github.com/pytorch/botorch/blob/8ecc903392b21ad9468c9b45f26b867d51378461/botorch/acquisition/acquisition.py#L149 I think it makes sense to keep things consistent. In addition, I think it is recommended for sobol sampling to use a sampling size which corresponds to basis 2.

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

Yes.

Pull Request resolved: #1535

Test Plan: Only default value is changed. No tests necessary.

Reviewed By: Balandat

Differential Revision: D41651210

Pulled By: saitcakmak

fbshipit-source-id: 12c0652eef45ee1493db2939ac3bcbacd191e213

Author: jduerholt

botorch/acquisition/utils.py

@@ -43,7 +43,7 @@ def get_acquisition_function(
     posterior_transform: Optional[PosteriorTransform] = None,
     X_pending: Optional[Tensor] = None,
     constraints: Optional[List[Callable[[Tensor], Tensor]]] = None,
-    mc_samples: int = 500,
+    mc_samples: int = 512,
     seed: Optional[int] = None,
     **kwargs,
 ) -> monte_carlo.MCAcquisitionFunction: