Improvements to Flax finetuning script
Author: marcvanzee
Pull request: yes

# What does this PR do?

- Ensures we actually use the `weight_decay` command-line argument
- Simplified `jax.value_and_grad` by removing the auxiliary (which we don't use)
- Simplified replication logic in eval step
- Fixes a bug in RNG handling. We weren’t splitting them appropriately before sharding them during training, which is not good practice, RNGs should always be split and not re-used, see: https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html#jax-prng

Note the new RNG handling affects the training accuracy, so I reran all experiments and report the new numbers, which aren't much different from the previous ones.

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