[caffe2] adds Cancel to OperatorBase and NetBase (#44145)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44145
## Motivation
* To be able to make C2 ops cancellable so we can safely exit.
* Some C2 operators are now blocking thus being non-cancellable. If an error
occurs we need to be able to safely stop all net execution so we can throw
the exception to the caller.
## Summary
* Adds `NetBase::Cancel()` to NetBase which iterates over the entire list of
operators and call Cancel.
* Cancel on all ops was added to Net since there's nothing Asyc specific about it.
* `AsyncSchedulingNet` calls parent Cancel.
* To preserve backwards compatibility, `AsyncSchedulingNet`'s Cancel still calls
`CancelAndFinishAsyncTasks` .
* Adds `Cancel()` to `OperatorBase`.
Reviewed By: dzhulgakov
Differential Revision: D23279202
fbshipit-source-id: e1bb0ff04a4e1393f935dbcac7c78c0baf728550
