Update transformers version for BERT pretraining

The example to pretrain BERT on Trainium is based on the original example from the AWS Neuron SDK [here](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/frameworks/torch/torch-neuronx/tutorials/training/bert.html#bert-large-compilation-limitations ).

This calls out a known issues with some versions of Transformers, especially with the BERT model downloading from the Hugging Face Hub.

I'm using transformers version 4.20.0, and now the compiler working.