It just happened to me in paperspace: https://nmf7d80p.gradient.paperspace.com/notebooks/fastbook/01_intro.ipynb
I searched for information about widgets in jupyter and gradient and found this:
https://ipywidgets.readthedocs.io/en/stable/examples/Widget%20Basics.html
Using that information I added the following line:
import ipywidgets as widgets
to the existing code so that it is now:
#hide_output
import ipywidgets as widgets
uploader = widgets.FileUpload()
uploader
and it ran and I could upload my image. So, if others are having that problem, try adding that import statement.