probability_below_threshold does assert_scalar for threshold while threshold can also be a TensorType

[ioananikova]

Describe the bug
The class description of probability_below_threshold acquisition function says that threshold can be a float or TensorType. However, at initialization with threshold a TensorType, the tf.debugging.assert_scalar fails and errors out.

To reproduce
Steps to reproduce the behaviour:

1. create a problem with the ProbabilityOfFeasibility acquisition function (or test with the probability_below_threshold class directly)
2. give as threshold argument a TensorType (this is useful when a modelstack is used and a different threshold is necessary for each constraint the model learns)
3. init will fail and say, e.g.: "ValueError: Expected scalar shape, saw shape: (1, 2)"
4. if you try this with a float as threshold, it does not error.

Include a minimal reproducible code example if relevant.

Expected behaviour
A TensorType should be supported. I commented the assertion and then it gave me correct results with the TensorType. So, the assertion should be changed to check for scalars and tensortypes.

System information

• OS: WSL
• Python version: 3.11.6
• Trieste version: 3.3.4 (but I saw that the latest version still has this assertion)
• TensorFlow version: 2.15.0
• GPflow version: 2.9.1

Additional context
Add any other context about the problem here.