[Feature Request] add task.unload() function to free up memory

[sgbaird]

On free tier, Colab runs out of RAM during the dummy benchmark: https://colab.research.google.com/drive/15YLOWHB_NkIIqKLO0ik784fsK2xJD08l?usp=sharing

A task.unload() function would delete task.df, and call gc.collect(), i.e.:

import gc
...
	task.load()
	...
	del task.df
	gc.collect()
...

This gets it a bit further (completes mp_e_form which it didn't before, but crashes at mp_gap). What other large objects could be safely deleted while preserving the benchmark results?