Make IndependentReparametrizationSampler create variables at init

[sam-willis]

Related issue(s)/PRs:

Summary

Make IndependentReparametrizationSampler create variables at init. Without this you cannot always save the sampler with dynamic shape (i.e. before sample has been called).
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Fully backwards compatible: yes / no

PR checklist

• The quality checks are all passing
• The bug case / new feature is covered by tests
• Any new features are well-documented (in docstrings or notebooks)

[sam-willis]

Closing as cannot support XLA compilation due to dynamic shape