Trainer: let generate pick its inputs
# What does this PR do?

Trainer's `predict_with_generate` seems to have been designed for an older version of `.generate()`, where manual selection of the inputs was needed. The current version of `.generate()` can do it on its own. This is particularly relevant for multimodal models, which can take more than one modality as input. As such, this PR removes the `.generate()` input selection logic from Trainer.

This PR is a requirement for Amazon's [MM-CoT](https://github.com/amazon-science/mm-cot).
_The documentation is not available anymore as the PR was closed or merged._