Remove correlation between the step size and the step direction (#2156) (#2290)

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## Motivation

This is a quick fix of #2156.

In the ```botorch.utils.sampling.sample_polytope()```, both the step size ```rands``` and direction ```Rs``` are currently sampled using the same seed value. This can lead to strong correlation and induce sampling bias, particularly with a small number of steps (e.g., ```n=1```), as illustrated in #2156. Adding ```+1``` to the seed value for ```Rs``` helps to avoid such behaviour.

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

Yes.

Pull Request resolved: #2290

Test Plan: Not sure if any unit tests needed for that change.

Reviewed By: esantorella

Differential Revision: D57208858

Pulled By: Balandat

fbshipit-source-id: 0653e751f4351a8f9ad2cf2625c93a3a5de7de63

Author: zankin

botorch/utils/sampling.py

@@ -249,8 +249,9 @@ def sample_polytope(
     # pre-sample samples from hypersphere
     d = x0.size(0)
     # uniform samples from unit ball in d dims
+    # increment seed by +1 to avoid correlation with step size, see #2156 for details
     Rs = sample_hypersphere(
-        d=d, n=n_tot, dtype=A.dtype, device=A.device, seed=seed
+        d=d, n=n_tot, dtype=A.dtype, device=A.device, seed=seed + 1
     ).unsqueeze(-1)
 
     # compute matprods in batch