fix(batch_norm): allow string cross_replica_axis for vmap support

[aditya161205]

Fixes NameError when using hk.BatchNorm with a named jax.vmap axis.

Problem

hk.BatchNorm previously assumed cross_replica_axis was always available (like in pmap). When used with vmap, this caused a NameError: unbound axis name during initialization because the named axis doesn't exist in the init context.

Fix

Added a try-except NameError block around the jax.lax.pmean calls.

• If the axis is unbound (e.g. during init), it now skips the sync and uses local stats.
• Inside vmap, it correctly syncs statistics across the batch dimension.

Verification

• Reproduced the crash locally using the script from #812.
• Verified fix: init no longer crashes, and vmap statistics match expected values (std ~1.0).
• Passed existing tests: haiku/_src/batch_norm_test.py

Closes #812
Closes #822

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