Hi,
This is a large computation for Google colab. There are some points in your training:

Resnet 101 is a very deep model and in takes too much memory and power to be trained.
GPUs on Google colab are shared on different VM instances, so you have only 16GB memory and a limited percentage of its power (Currently T4 GPU).
I suggest to add a loss visualizer or print loss for every epoch and batch to see where you get stuck and what is gooing on. TensorboardX is available to help you through this.

Good luck