kvcache: Remove special case for reservation mask

We currently short circuit generation of the cache mask and just
generate an empty tensor of the correct size. However, in some
cases, this can also skip a cast operation. This can result in the
worst case graph being not fully worst case.

We don't actually need the fast path for mask generation, so it's
better to just use the normal code path.

kvcache/causal.go

@@ -40,11 +40,6 @@ type Causal struct {
 
 	// ** current forward pass **
 
-	// curReserve indicates that this forward pass is only for
-	// memory reservation and we should not update our metadata
-	// based on it.
-	curReserve bool
-
 	// the active layer for Get and Put
 	curLayer int
 
@@ -206,13 +201,12 @@ func (c *Causal) Close() {
 }
 
 func (c *Causal) StartForward(ctx ml.Context, batch input.Batch, reserve bool) error {
-	c.curReserve = reserve
 	c.curBatchSize = len(batch.Positions)
 	c.curSequences = batch.Sequences
 	c.curPositions = batch.Positions
 	c.opts.Except = nil
 
-	if !c.curReserve {
+	if !reserve {
 		c.updateSlidingWindow()
 
 		var err error
@@ -379,10 +373,6 @@ func (c *Causal) buildMask(ctx ml.Context) ml.Tensor {
 
 	length := c.curCellRange.max - c.curCellRange.min + 1
 
-	if c.curReserve {
-		return ctx.Input().Empty(c.config.MaskDType, length, batchSize)
-	}
-
 	mask := make([]float32, batchSize*length)
 
 	for i := range c.curBatchSize {