Fixing plot gallery examples

[glemaitre]

Description

There is a need to use plt.tight_layout() to avoid problem with the font or even reducing the font.
The issues are for the following plots:

• http://contrib.scikit-learn.org/imbalanced-learn/auto_examples/under-sampling/plot_allknn.html#sphx-glr-auto-examples-under-sampling-plot-allknn-py
• http://contrib.scikit-learn.org/imbalanced-learn/auto_examples/under-sampling/plot_repeated_edited_nearest_neighbours.html#sphx-glr-auto-examples-under-sampling-plot-repeated-edited-nearest-neighbours-py
• http://contrib.scikit-learn.org/imbalanced-learn/auto_examples/under-sampling/plot_instance_hardness_threshold.html#sphx-glr-auto-examples-under-sampling-plot-instance-hardness-threshold-py
• http://contrib.scikit-learn.org/imbalanced-learn/auto_examples/datasets/plot_make_imbalance.html#sphx-glr-auto-examples-datasets-plot-make-imbalance-py

Steps/Code to Reproduce

Expected Results

Actual Results

Versions