DL101 Week 1 interp.plot_top_losses plots only one image #64

Describe the bug

In the DL101 notebook, the interp.plot_top_losses(5, nrows=5) line will only return one image.
It is critical to the notebook running correctly for students.

To Reproduce
Steps to reproduce the behavior or code you have used

Run the notebook, train the model, then run this cell

interp.plot_top_losses(5, nrows=5)

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
  Ubuntu
• Browser [e.g. chrome, safari]
  Firefox
• Version [e.g. 22]

Additional context

This has been mentioned by several students, and is important to their experience.