Use c10::variant-based enums for EmbeddingBag mode (#28330)
Summary:
This PR is BC-breaking in the following way:
Previous, we require the use of `std::string` to specify the mode for `EmbeddingBag`. After this PR, we use variant-based enums such as `torch::kSum` / `torch::kMean` / `torch::kMax` to specify the mode for `EmbeddingBag`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28330
Differential Revision: D18127116
Pulled By: yf225
fbshipit-source-id: 15cd86c764777f4d399587be92cda15b6ce8524b
