### What this PR does / why we need it?
Currently, we call lookup_client for looking up token hit in KV Pool,
however, when token length < block size, the key will be empty and there
is no point to lookup in KV Pool backend since there will never be a
hit.
Hence, add early return in `get_num_new_matched_tokens` when `token_len`
< `block_size`
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- vLLM version: v0.16.0
- vLLM main:
4034c3d32e
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Signed-off-by: Pz1116 <zpbzpb123123@gmail.com>
Co-authored-by: fems14 <1804143737@qq.com>