[Kernel] sliding window support in paged_attention_v1/v2 kernels

[mmoskal]

The paged attention (v1 and v2) decode kernel does not support sliding window natively - the way it works now it just takes all the blocks passed in (up to seq_len). With v1 manager, the sliding window uses blocks in a "ring buffer" fashion, so this is not a problem. With the new block manager (see #4545) we need potentially to start attention computation in the middle of a block, otherwise we pay attention to a few tokens too many. It doesn't seem to affect this test though.

Related: #3385

This is just a rough draft - didn't even try to compile.

@cadedaniel is there another issue for this already?

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[rkooo567]

cc @zhuohan123 (great timing!)

[rkooo567]

btw, we need to make sure this can be compatible with this PR #4681, that we plan to merge soon.. I think this can make things much cleaner