In a previous commit, we chose to override the TorchVision ResNet
models with our own modified versions of ResNet because collecting
activation statistics after ReLU layers requires that each ReLU
instance is only used once in the graph.

However, overriding TorchVision models means that if someone wants
to directly access the TorchVision model dictionary, he/she won't
find the ResNet models in the dictionary keys.  This is not very
friendly and we should not break imported modules.

This commit overrides the TorchVision ResNet models when accessed
via the distiller.models.create_model API, w/o changing the
TorchVision dictionary.