Not a solution, but note that after the installation of sentencepiece (0.1.95) the error message changes to:
ImportError: 
PegasusConverter requires the protobuf library but it was not found in your environment. Checkout the instructions on the
installation page of its repo: https://github.com/protocolbuffers/protobuf/tree/master/python#installation and follow the ones
that match your environment.

So if you want to use that specific tokenizer, you should probably install protobuf.
This is, however, not related to your GPU memory issues. As @neuralpat pointed out, the memory issue should be tackled by decreasing the batch size.