Migrate AttentionOp to use NNX

[bvandermoon]

Description

Update the AttentionOp module to use NNX instead of Linen.

AttentionOp has no dependencies/params. Attention and Gpt3MultiHeadAttention both depend on it.

Tests

Successfully trained MaxText base on a TPU VM for 10 steps:

python3 -m MaxText.train MaxText/configs/base.yml \
    run_name=<run_name> \
    base_output_directory=gs://<gcs_bucket> \
    dataset_type=synthetic \
    steps=10

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.

[richjames0]

lgtm with thanks