Process was killed

Hi,

I tried to run the following code

```python
tpot = TPOTRegressor(generations=500, population_size=50, verbosity=2, n_jobs=16, use_dask=True)
tpot.fit(X_train, y_train)
tpot.export("tpot_{db}.py".format(db=db))
print(db, tpot.score(X_train, y_train), tpot.fitted_pipeline
```
but, after few generations, the script was killed by system (literally "killed" is the only message depicted in console).

by running **dmesg**, I got

```
[169102.053126] Out of memory: Killed process 446321 (python) total-vm:194467732kB, anon-rss:107278388kB, file-rss:0kB, shmem-rss:4kB, UID:1000 pgtables:225528kB oom_score_adj:0
[169105.064216] oom_reaper: reaped process 446321 (python), now anon-rss:0kB, file-rss:0kB, shmem-rss:4kB
```

The dataset is small (232 rows and 108 columns). A similar program in AutoSklearn consumes about 18GB ram. Is it possible that there is a memory leak?

I'm using Python=3.7.9 and tpot=0.11.7. I tried n_jobs=1, use_dask=False, but the error remains the same.