Add support for other pooling methods in python backend

[OskarLiew]

Feature request

The python backend currently only supports CLS pooling

Motivation

I am currently deploying on intel gaudi hardware, and their fork of TEI currently only has support for the python backend. The model that I use has half the error rate when using mean pooling. Rather than only contributing to their fork I thought I could submit the update upstream.

Your contribution

I have made code changes and will publish a PR right away when I am done writing this.