Base models and notebooks for general IPU embeddings model

[arsalanu]

Support for multiple embeddings models (e5, mpnet, t5 (pending))
Idea is to create a general embeddings class which supports the pool and norm stage of embeddings model. These models use encoder stages from various base models (bert, t5, mpnet) which we don't support the direct use of with pipelining. I've added these in this folder for now, but they may need to be in the source lib if integrated.

Seemed like not a great idea to create new pipelined classes for all of these models, as they inherit from existing models we support and would just be an extra couple of lines which are common amongst them, so makes more sense to have a generic wrapper for embeddings, which instantiates the encoder stages through AutoModel as currently shown in HF.

The inference setup is manually outlined, but may be more user-friendly to create a pipeline for these eventually.

MPNet modeling file only includes the base model and maskedLM model, not tested for pretraining, only for embeddings.

notebook will likely be restructured

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[arsalanu]

relies on #437 being merged first