Add bf16 benchmark for GPT3-175B

[bvandermoon]

Description

Add a new Trillium benchmark that runs GPT3-175B with BF16. I ran this on v6e-256 and got 392 TFLOPs.

This change is needed to support adding a reproducible Trillium recipe.

Tests

Ran this on one slice of v6e-256 using the following command:

python3 benchmarks/benchmark_runner.py xpk \
    --project=${PROJECT} \
    --zone=${ZONE} \
    --device_type=v6e-256 \
    --num_slices=1  \
    --cluster_name=${CLUSTER_NAME} \
    --base_output_directory=${OUTPUT_DIR} \
    --model_name="gpt_3_175b_bf16" \
    --base_docker_image=maxtext_base_image

I got the perf described above.

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.