Rename GenerateMode to DownloadMode
COLLABORATOR
Thanks! Used here: https://github.com/huggingface/datasets-preview-backend/blob/main/src/datasets_preview_backend/models/dataset.py#L26 :) 
2022-02-22T13:57:24Z
https://github.com/huggingface/datasets/pull/3759#issuecomment-1047822901
https://api.github.com/repos/huggingface/datasets/issues/3759
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2022-02-22T13:57:24Z
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This PR:
- Renames `GenerateMode` to `DownloadMode`
- Implements `DeprecatedEnum`
- Deprecates `GenerateMode`

Close #769.