Update examples to use newer torchtext APIs

[erip]

I'm in the process of updating the pytorch benchmarks repo to use newer torchtext APIs. These benchmarks use an adapted version of this example notebook. If it makes sense, I'm happy to submit a PR to similarly update the examples here (and in other notebooks where relevant).

[srush]

Would love that.

[erip]

Great. There are some caveats about parity between features from legacy torchtext APIs and new ones, but they likely won't be of consequence for this repo. The biggest change is how bucketing works intra-batch now that BucketIterator is gone. Once the current PR has gotten reviewed and lands I'll cut a PR here too.