Relax dependency bounds and make comet optional

[smirnovlad]

Summary

• Remove spacy<3.8.0 upper bound: allows pip to resolve newer thinc / numpy>=2.0, which is required for compatibility with vLLM and other packages that need numpy 2.x
• Pin transformers>=4.50.0,<5: transformers 5.x introduces breaking API changes (removed BeamScorer, renamed output classes, removed batch_encode_plus). This pin keeps the current code working while we prepare a separate forward-compatibility PR.
• Make unbabel-comet optional: commented out from requirements.txt, lazy-imported in comet.py, with try/except in __init__.py. This removes the hard dependency that was pulling in conflicting transformers version constraints.
• Updated README with an "Optional dependencies" section explaining how to install unbabel-comet separately.

Motivation

This is a prerequisite for publishing ThinkBooster as a PyPI package — lm-polygraph must be installable without manual patching of dependency pins.

What this PR does NOT include

Transformers 5.x forward-compatibility code changes (try/except import fallbacks, batch_encode_plus replacements, logits sanitizer). Those will come in a separate PR.

Test plan

• CI passes with existing tests (no code changes, only dependency resolution)
• pip install in a clean environment resolves without conflicts
• COMET metric works when unbabel-comet is installed separately
• COMET-dependent code gracefully skips when unbabel-comet is not installed

[smirnovlad]

unbabel-comet pins numpy<2.0.0,>=1.20.0 — this conflicts with vLLM which requires numpy>=2.0. That's the reason we strip comet in ThinkBooster's setup.sh (sed -i '/unbabel-comet/d').

The transformers<5 constraint from comet is indeed fine since we already pin that ourselves. But the numpy pin is the actual blocker.

Making comet optional here lets lm-polygraph install cleanly alongside vLLM without needing post-install sed hacks.

[smirnovlad]

Superseded by #446 which now includes both dependency changes and transformers 5.x compat code.