- previously, vec_ in DeviceShard wasn't updated on copy; as a result, the shards continued to refer to the old HostDeviceVectorImpl object, which resulted in a dangling pointer once that object was deallocated
- previously, vec_ in DeviceShard wasn't updated on copy; as a result, the shards continued to refer to the old HostDeviceVectorImpl object, which resulted in a dangling pointer once that object was deallocated