Fix a stream race condition bug exposed in matmul
Fix a stream race condition bug exposed in matmul

- cuda::child_stream must wait for its parent stream after
  construction to ensure that prior dependencies have completed.
- This bug was observed in the linalg tests only when another
  GPU-intensive process was running in the background. These
  circumstances allowed the matmul kernel and a prior initialization
  kernel to race, producing incorrect/non-deterministic results.

ledatelescope/bifrost,ledatelescope/bifrost,ledatelescope/bifrost,ledatelescope/bifrost