Dataset leak issue with at least Tabular

[muellerzr]

For the last day @jph00 and I have been fighting references to datasets inside of tabular and around fastai in general. We fixed two, referenced here and here and this issue partially originated from this flag: #2945

Through the use of the pympler library (docs here ) I was able to narrow down the fact that we have two extraneous DataFrames still being exported via our load_learner. It's a bit more of a complex problem so here is a google colab notebook recreating what we've done:

https://gist.github.com/muellerzr/c126e75b0265f88c3baa1ea50a4dfe6b

[jph00]

Should be fixed now.