Enable FSDP for Prompt Tuning via Peft

[alex-jw-brooks]

Description

As a user of prompt tuning, I want to be able to leverage multiple GPUs at train time!

Discussion

Extends #175 to leverage PyTorch FSDP; there may be some refactoring to consolidate with fine tuning FSDP configuration / dataset stuff, which may be part of this story, or left to a follow up. We will decide once we see how Peft / trainer integration goes.

Acceptance Criteria

• Unit tests cover new/changed code
• Examples build against new/changed code
• READMEs are updated
• Type of semantic version change is identified

[chakrn]

@gkumbhat can you also link the relevant PR to this one?