[Stacked PR 5/5] Add GDN backward pass 8k and 64k latency and memory benchmark suite

- Add standalone 8k and 64k Ghostfish Cloud TPU latency and memory benchmark suite (gdn_benchmark_test.py).
- Provide comparative benchmarking between Pure JAX remat and Decoupled GDN Pallas backward pass kernel across sequence lengths up to 64k.

Author: Rohan-Bierneni