Plots should be explicitly closed in add_cdf_image_summary

[ethereon]

In the function add_cdf_image_summary within object_detection/utils/visualization_utils.py:

The CDF plots created using matplotlib (for instance, when training an object detector with focal loss) are not properly closed. Consequently, they aren't garbage collected and result in the following warning during training:

/usr/local/lib/python2.7/dist-packages/matplotlib/pyplot.py:524: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (matplotlib.pyplot.figure) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam figure.max_open_warning)

Inserting plt.close(fig) before returning the image resolves the issue.

[tensorflowbutler]

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[Harshini-Gadige]

Closing as this is in "awaiting response" for more than a week. Feel free to add more information and we will reopen. Thanks !