Kimi-K2 release update

[snehalv2002]

Description

update the README with Kimi-K2 release

Tests

Best pretraining MFU currently 6.43%, on TPU7x-512 (256 chips), optimizer=Muon, EP=8, FSDP=64, PDBS=16, and SEQ_LEN=1024

Checklist

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

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• 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, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.