feat: make datasets and accelerate optional dependencies

[andrea-fasoli]

The version caps datasets and accelerate bring (e.g. fsspec<=2026.2.0 from datasets 4.x) is blocking vulnerability updates.
Neither package is needed by the aiu_addons, which is the only part of fms-mo that FMS loads.
This PR make these two packages installation optional.

Related issues

Closes #212

Changes

• pyproject.toml: moved datasets into a new data extra and accelerate into a new launcher extra. Both are included in opt, examples and test, so the Docker image and CI are unchanged.
• run_quant.py, dq.py, utils/calib_data.py: if datasets is missing, importing these modules now raises an ImportError that says to install fms-model-optimizer[data].
• README.md: documented the new extras.

Breaking change

With a plain pip install fms-model-optimizer, running run_quant or direct quantization now requires the [data] extra.

Verification

• Clean install without extras: datasets and accelerate are not installed. All aiu_addons modules import, and neither package is loaded afterwards.
• The import guards raise the expected error.
• Unit tests are passing.

[chichun-charlie-liu]

lgtm, thx!