Transformers 5.20.2

Patch release: v5.20.2

This patch notably solves an issue where some inputs could loose their original type inside prepare_inputs_for_generation in generate, leading to problems with multimodal models. It also fixes some other issues listed on the below commits.

It contains the following commits:

• Fix inputs dict casting in generate (#49731) by @Cyrilvallez
• Fix zero expert gradients in bfloat16 with expert parallelism and ep_size < fsdp_size (#49688) by @qgallouedec
• Declare the input modalities of the VLMs that report text only (#49677) by @qgallouedec
• Avoid a host-to-device copy in eager_mask (#49684) by @hmellor