Using CancelBackwardException introduced in #3644 to create own backward pass

[olivialocke]

I have a callback that implements a custom backward pass, thus, would like to attach a RaiseBackwardException (CancelBackwardException) in the implementation as mentioned in #3644 .

However, while 'CancelStepException' and 'CancelBatchException' etc can be imported from fastai.callback, 'CancelBackwardException' cannot.

Is there a specific way to import / how should the RaiseBackwardException be attached in the implementation?

Thanks!

[olivialocke]

The version needed to be updated (from fastai 2.6.3 to 2.7.5).