chore(model,prompts): add type annotations to module-level functions and __init__ methods

[Koushik-Salammagari]

What does this PR do?

Adds missing Python 3.10+ style type annotations to litgpt/model.py and litgpt/prompts.py. These two files had the second and third highest counts of unannotated functions after utils.py (which was covered in #2237).

litgpt/model.py

Function	Added
yarn_get_mscale	scale: float, mscale: float, -> float
batched_index_select	t: torch.Tensor, dim: int, idx: torch.Tensor, -> torch.Tensor
batched_index_copy_	t: torch.Tensor, dim: int, idx: torch.Tensor, val: torch.Tensor, -> torch.Tensor
find_correction_dim (nested in build_rope_cache)	num_rotations: float, dim: int, base_val: float, max_pos: int, -> float

litgpt/prompts.py

Added -> None return annotation to all __init__ methods that were missing it:

Class
ChatML
Qwen2_5
Qwen2_5_Math
QwQ
Qwen3
SmolLM2
Salamandra

No new imports needed in either file.

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.