make threefry split and fold_in symmetric
#13341
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Namely, make it so that `split(key, n)[i]` equals `fold_in(key, i)` for any key and for `0 <= i < n`. This change affects the observed random bits for a fixed key (indirectly through splits and folds), so here we guard it behind `jax.config.jax_threefry_partitionable`. It's not described very well by the flag name, but it makes for a simple way to bundle together several random-bit-altering changes as part of the same upgrade cycle.
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Namely, make it so that
split(key, n)[i]equalsfold_in(key, i)for any key and for0 <= i < n.This change affects the observed random bits for a fixed key (indirectly through splits and folds), so here we guard it behind
jax.config.jax_threefry_partitionable. It's not described very well by the flag name, but it makes for a simple way to bundle together several random-bit-altering changes as part of the same upgrade cycle.Fixes #7708, up to the
jax_threefry_partitionableupgrade, with a test to confirm.