expected_calibration_error does not crash with VariableArray objects

[Kucharssim]

PR #321 broke the ece because the function made an incorrect assumption that if the inputs are not type of np.ndarray, they must be tf.Tensor with a .numpy() method.

The fix can be tested against the model comparison example notebook.

[paul-buerkner]

Looks good. Thank you! Waiting for checks to pass and will then merge.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/utils/comp_utils.py	33.33%	2 Missing ⚠️

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Files with missing lines	Coverage Δ
bayesflow/utils/comp_utils.py	17.39% <33.33%> (+4.89%)	⬆️