Don't upload unused operation outputs

[brollb]

Operation outputs are uploaded after the job completes so it is available for any subsequent jobs. However, this is particularly slow and inefficient when the output is not used. It would be good to update the operation to only upload used operations.

[brollb]

In the future, this could be used to send the data directly to the next operation for streaming or as an optimization for compute backends. For example, the server compute backend could skip uploading entirely between operations and simply place it in a tmp directory.