add from_pretrained method to EmbeddingBag (#15273)
Summary:
The `EmbeddingBag` module does not include a `from_pretrained` method like the `Embedding` module. I added it for consistency between the two modules.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15273
Differential Revision: D13547842
Pulled By: soumith
fbshipit-source-id: 8ffde51ff0c1e8fc8310263b6f375da88089ff7d
