runner: rename stop to cancel

[d4l3k]

Description

Currently the API to cancel a job is named stop in runner and cancel in the CLI and scheduler APIs. We should update these so they're all the same. Due to backwards compatibility reasons I don't think we can eliminate stop entirely but we can mark it as deprecated and add cancel as the primary path.

https://github.com/pytorch/torchx/blob/main/torchx/runner/api.py#L407-L424