Fix a bug for `CallbackHandler.callback_list`#8052

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LysandreJik merged 3 commits intohuggingface:masterfrom

harupy:fix-callback_list

Oct 27, 2020

Commits on Oct 26, 2020

Fix callback_list
harupy
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Commits on Oct 27, 2020

Add test
harupy
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Fix test
harupy
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