Add classical training and episodic training notebooks to colab

[tnwei]

First of all thanks for creating this repo! I have found it useful, and would like to contribute.

Out of the three example notebooks in the repo, only one directly works on Google Colab. In this PR, I've slightly modified the other two notebooks (notebooks/episodic_training.ipynb and notebooks/classical_training.ipynb) so that they can also be directly opened in Colab, which should make it even more convenient for first time users to try them out.

[tnwei]

Sorry just got the time to get back to you, just put in the changes requested @ebennequin

[ebennequin]

Thank you @tnwei I'm merging this :)