Add missing type hints in utils/memory.py

[RudrenduPaul]

What this PR does

Adds missing type hints to two functions in src/accelerate/utils/memory.py:

• clear_device_cache(garbage_collection: bool = False) -> None
• release_memory(*objects) -> list

This follows the same self-identified typing-consistency pattern as my prior
PRs in this repo (#4123, #4124, #4125) — no linked issue, just filling in
type hints that were missing while the rest of the module (and the file's
other two functions, should_reduce_batch_size and
find_executable_batch_size) already had them.

Functions updated

• clear_device_cache: garbage_collection is only ever called with bool
  values (True/False) at every call site in the codebase, and the
  function has no return statement, so -> None is accurate.
• release_memory: builds a list from *objects, sets every element to
  None, and returns that list — verified by reading the full function
  body, so -> list is accurate. The variadic *objects parameter itself
  is intentionally left unannotated (it accepts arbitrary object types to
  release from memory), consistent with how *args/**kwargs were left
  unannotated in my earlier hooks.py (Add module: nn.Module type hints to ModelHook lifecycle methods #4124) and logging.py (Add level and msg type hints to MultiProcessAdapter.log #4125) PRs.

Functions intentionally left unchanged

• should_reduce_batch_size and find_executable_batch_size already have
  complete type hints on every parameter in this file — no changes needed.

No behaviour change

Type-hint-only change. No logic was modified.

Testing

• ruff check src/accelerate/utils/memory.py — all checks pass.
• ruff format --check src/accelerate/utils/memory.py — passes, file
  already formatted.
• python3 -c "import ast; ast.parse(...)" on the modified file — passes.

Note: Claude Code was used to assist in drafting this change. All
changes were reviewed by the submitter.