fix cancellation bug in SweepablePipelineRunner && fix object null exception in AutoML v1.0 regression API

[LittleLittleCloud]

We are excited to review your PR.

So we can do the best job, please check:

• There's a descriptive title that will make sense to other developers some time from now.
• There's associated issues. All PR's should have issue(s) associated - unless a trivial self-evident change such as fixing a typo. You can use the format Fixes #nnnn in your description to cause GitHub to automatically close the issue(s) when your PR is merged.
• Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• You have included any necessary tests in the same PR.

why cancellation doesn't work for SweepablePipelineRunner

SweepablePipelineRunner cancels running trial by calling context.CancelExecution(). When starting a trial, SweepablePipelineRunner first registers a context.CancelExecution() callback on cancellationtoken which will be triggered if cancellationtoken get cancelled. Then blocking on the running task until trial completed.The register callback will be unregistered after running trial completed through using statement.

The cause of cancellation doesn't work is SweepablePipelineRunner doesn't actually block on running trial. It simply create a wrapped task that kick off a trial, and return that task immediately. After the wrapped task is returned, the context.CancelExecution() will be unregistered and the running trial won't be cancelled any more.

The fix is simply change SweepablePipelineRunner to wait until the trial task is completed before return.

[LittleLittleCloud]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[JakeRadMSFT]

@LittleLittleCloud - Can you update the description a bit :)

[LittleLittleCloud]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).