imagenet_r dataset added

[ranarag]

This change resolves #284 by adding the option to load ImageNet-R dataset and also includes the pytest for the same.

[TLESORT]

Hi @ranarag ,
Thanks for you PR ! :)
Sorry for not looking at your PR earlier.
If I understand correctly you want to reproduce an existing scenario from 'https://github.com/GT-RIPL/CODA-Prompt' ?

Maybe as Imagenet_R was not proposed in https://github.com/GT-RIPL/CODA-Prompt but in another paper, it would be better to only stick to the original data in the dataset class.
The data specific to the scenario could be provided in a scenario class similarly to ALMA in https://github.com/Continvvm/continuum/blob/master/continuum/scenarios/specific_scenarios.py file.

Also, adding a description of the scenario in the doc would be nice :)

have a nice day
Timothée