kvcache: Sliding window cache only needs a single batch total

When computing the size of the cache for sliding window attention,
we don't need to multiple the batch size by the number of parallel
sequences - the batch size is constant.

This also simplifies the check for whether to allocate the cache
size based on capacity or window size as the batch size is already
incorporated into the capacity when handled by the runner.

kvcache/causal.go

@@ -119,10 +119,10 @@ func (c *Causal) Init(backend ml.Backend, dtype ml.DType, maxSequences, capacity
 	}
 
 	var cacheSize int
-	if c.windowSize == math.MaxInt32 || capacity < int(c.windowSize)+maxBatch {
+	if c.windowSize == math.MaxInt32 || capacity < int(c.windowSize) {
 		cacheSize = maxSequences * capacity
 	} else {
-		cacheSize = maxSequences * (int(c.windowSize) + maxBatch)
+		cacheSize = (maxSequences * int(c.windowSize)) + maxBatch
 	}
 	cacheSize = roundUp(cacheSize, c.config.CachePadding)
 	c.cells = make([]cacheCell, cacheSize)