[Hardware][TPU] improve kv cache update performance in prefill

[yaochengji]

TPU is not good at scatter(or index_copy in pytorch), this PR modify the slot_mapping logic to make the kv cache get updated at a page granularity, rather than a token granularity.

Based on the profling, the latency of a prefill step with bs=1 and seq_len=1024 drops from 82ms to 37ms. The kv_cache_update has a ~17x performance improvement.

Potential risk: this change needs the pages assigned to the prefill to be empty.

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[yaochengji]

Seems the CI failure has nothing to do with my change.

pythonic_tool_parser.py:171] SyntaxError: ':' expected after dictionary key

huggingface_hub.errors.LocalEntryNotFoundError: An error happened while trying to locate the file on the Hub and we cannot find the requested files in the local cache. Please check your connection and try again or make sure your Internet connection is on.