kvcache: Don't shift empty batches

When we context shift, we delete half the context and apply RoPE
with an offset to the other half. We used to RoPE across the entire
context in a single pass with a zero offset for the deleted
section. With the change to shifting in batches, we can skip any
batches where all of the offsets would be zero. This typically
reduces the number of operations by half.

kvcache/causal.go

@@ -646,18 +646,31 @@ func (c *Causal) shift(seq int, beginIndex, offset int32) error {
 	seqRange := c.cellRanges[seq]
 
 	for start := seqRange.min; start <= seqRange.max; start += c.maxBatch {
-		ctx := c.backend.NewContext()
-
 		size := min(seqRange.max-start+1, c.maxBatch)
 		offsets := make([]int32, size)
+
+		var batchFirst, batchLast int
+
+		batchFirst = -1
 		for i := range offsets {
 			cell := c.cells[start+i]
 
 			if slices.Contains(cell.sequences, seq) && cell.pos >= beginIndex {
 				offsets[i] = offset
+				if batchFirst < 0 {
+					batchFirst = i
+				}
+				batchLast = i
 			}
 		}
 
+		if batchFirst < 0 {
+			continue
+		}
+
+		offsets = offsets[batchFirst : batchLast+1]
+
+		ctx := c.backend.NewContext()
 		kShift := ctx.Input().FromIntSlice(offsets, len(offsets))
 
 		for i, key := range c.keys {
@@ -669,10 +682,10 @@ func (c *Causal) shift(seq int, beginIndex, offset int32) error {
 			numKVHeads := key.Dim(1)
 			rowSize := key.Stride(2)
 
-			key = key.View(ctx, rowSize*start,
+			key = key.View(ctx, rowSize*(start+batchFirst),
 				kHeadDim, key.Stride(1),
 				numKVHeads, key.Stride(2),
-				size,
+				len(offsets),
 			)
 
 			roped, err := c.shiftFn(ctx, i, key, kShift)