chore(data): add return type annotations to FLAN dataloader methods

[Koushik-Salammagari]

What does this PR do?

Adds missing return type annotations to three methods in litgpt/data/flan.py that were previously unannotated:

Method	Added
FLAN.__post_init__	-> None
FLAN.train_dataloader	-> DataLoader
FLAN.val_dataloader	-> DataLoader

DataLoader was already imported at the top of the file, so no new imports are needed.

This follows the pattern established in #2221 (which bumped ruff target-version to py310) and continues the type annotation work started in #2237.

Before submitting

• Was this discussed/approved via a GitHub issue? Follows the type-annotation pattern from chore(ruff): bump target-version to py310 and update type annotations #2221 and chore(utils): add type annotations to public functions in utils.py #2237.
• Did you make sure to update the docs? N/A — annotation-only change.
• Did you write any new unit tests? N/A — no behavioural change.