Add missing -> bool return type hints in utils/imports.py

[RudrenduPaul]

What this PR does

Adds -> bool return type hints to nine is_*_available device/feature-check
helper functions in src/accelerate/utils/imports.py, matching the existing
precedent set by is_torch_distributed_available() -> bool in the same file.

This is a self-identified typing-consistency fix (no linked issue) — the file
already establishes the -> bool convention for one helper but several
sibling functions with identical return semantics were left unannotated.

Functions updated

• is_torch_xla_available
• is_bf16_available
• is_mps_available
• is_mlu_available
• is_musa_available
• is_npu_available
• is_sdaa_available
• is_hpu_available
• is_xpu_available

Each of these functions returns only bool values on every code path
(verified by reading the full function body of each before annotating).

Scope note: torchao_required / deepspeed_required intentionally excluded

Both are decorator factories that take a func argument and return a
wrapper callable (via functools.wraps) — they never return bool.
Adding -> bool to them would be a factually incorrect type hint, so they
were left unannotated in this PR rather than introduce an inaccurate
signature. Happy to follow up with a correct Callable-based annotation
for those two in a separate PR if maintainers want it.

No behaviour change

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

Testing

• ruff check src/accelerate/utils/imports.py — 6 pre-existing findings,
  none on changed lines, none introduced by this change (verified against
  upstream main before/after).
• ruff format --check src/accelerate/utils/imports.py — passes, file
  already formatted.
• python3 -m py_compile on the modified file — passes.

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