[caffe2] adds Cancel to SafeDequeueBlobsOp and SafeEnqueueBlobsOp (#45177)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45177
## 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
* 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.
Test Plan:
## Unit test added to verify that queue ops propagate errors
```
buck test caffe2/caffe2/python:hypothesis_test -- test_safe_dequeue_blob__raises_exception_when_hang --stress-runs 1000
```
```
Summary
Pass: 1000
ListingSuccess: 1
```
Reviewed By: d4l3k
Differential Revision: D23846967
fbshipit-source-id: c7ddd63259e033ed0bed9df8e1b315f87bf59394
