Fix prefill OOM error in the case of large page size

[xiezhq-hermann]

Motivation

In the case of large page size, current scheduler might underestimate the need for memory, thus causing OOM error, e.g.,:

RuntimeError: Prefill out of memory. Try to lower your batch size.Try to allocate 1444 tokens.Avaliable tokens:5120self.token to kv pool allocator.available size()=5120self.tree cache.evictable size()=0

Modifications

Align the real memory need for page size > 1.

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