Fix pipeline parallel with multimodal models with the Transformers modelling backend

[hmellor]

In Transformers v4 tie_word_embeddings is found in the text model config, but in v5 it's found in the top level config. The Transformers modelling backend now checks both places.

Base.pipeline_parallel was not able to apply the pipeline parallel plan to multimodal models because the plan referred to nested modules (text_model.layers for example). This Transformers modelling backend now uses attrgetter (and a custom inverse attrsetter) to make sure that nested modules can still be accessed.

[gemini-code-assist]

Code Review

This pull request introduces two main improvements. First, it adds support for pipeline parallelism in multimodal models by using attrgetter and a new attrsetter helper to handle nested modules, which is a clean and effective solution. Second, it refactors the logic for checking tie_word_embeddings to support both Transformers v4 and v5 configurations, centralizing this logic in a new _get_tie_word_embeddings method. The changes are well-implemented, correctly address the described issues, and improve the overall robustness and feature set of the Transformers modeling backend. The code is clear and the approach is solid.