mypy fails to check Gradio frontend

The current version of Gradio does not export symbols correctly, causing mypy checks to fail. We can either submit a fix upstream or temporarily ignore the affected lines. ``` error: Module "gradio" does not explicitly export attribute "Checkbox" [attr-defined] error: Module "gradio" does not explicitly export attribute "Dataframe" [attr-defined] error: Module "gradio" does not explicitly export attribute "Dropdown" [attr-defined] error: Module "gradio" does not explicitly export attribute "Error" [attr-defined] error: Module "gradio" does not explicitly export attribute "File" [attr-defined] error: Module "gradio" does not explicitly export attribute "Interface" [attr-defined] error: Module "gradio" does not explicitly export attribute "Plot" [attr-defined] error: Name "gr.Interface" is not defined [name-defined] ```