Couple of numpy-related fixes

[davidwilby]

📝 Description

We have a few tests routinely failing at the moment for a handful of reasons mostly relating to numpy 2.

One such error is due to DeepSensor's dependency stheno not yet pinning the version of fdm which repairs some of the numpy compatibility bugs. I've raised a PR in that project here: wesselb/stheno#27

In this PR there are a couple of corrections to alleviate test failures:

1. Use of np.product in some tests, this was changed in something like numpy 1.16 to np.prod.
2. The type of encoder_scales was being set to a numpy float32 with numpy 2 (possible relating to https://numpy.org/devdocs/numpy_2_0_migration_guide.html#changes-to-numpy-data-type-promotion) which was causing the saving and loading test to fail. I've added an int() but unsure if this is appropriate/ideal, though all of the tests pass locally.

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