[caffe2] adds Cancel to SafeDequeueBlobsOp and SafeEnqueueBlobsOp (#44495)
Summary:
## 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.
* When an error occurs in a net or it got cancelled, running ops will have the
`Cancel` method called.
* This diff adds `Cancel` method to the `SafeEnqueueBlobsOp`
and `SafeDequeueBlobsOp` to have the call queue->close() to force all the
blocking ops to return.
* Adds unit test that verified the error propagation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44495
Test Plan:
## Unit Test added to verify that queue ops propagate errors
```
buck test caffe2/caffe2/python:hypothesis_test
```
Reviewed By: dzhulgakov
Differential Revision: D23236088
Pulled By: dahsh
fbshipit-source-id: daa90d9ee32483fb51195e269a52cf5987bb0a5a
