[Flax] Fix incomplete batches in example scripts (#17863)
* [Flax] Fix incomplete batches in example scripts
* fix dataloader batching
* convert jnp batch idxs to np array
* add missing `pad_shard_unpad` to final prediction generate step
* only `pad_shard_unpad` at inference time
* merge conflicts
* remove incomplete batch step from eval
* fix run_qa.py
* add `pad_shard_unpad` to run_flax_ner.py
* add `pad_shard_unpad` to run_flax_glue.py
* add `pad_shard_unpad` to run_image_classification.py
* make style
* fix mlm flax eval batches
* remove redundant imports