fix: loosen tokenizers pin to resolve dependency conflict with transformers

The exact pin `tokenizers==0.20.3` conflicts with `transformers>=4.48.0`,
which requires `tokenizers>=0.21`. This makes `pip install -r requirements.txt`
fail with an unresolvable dependency error.

Changed to `tokenizers>=0.20.3` to allow pip/uv to resolve a compatible
version. Verified that the full requirements.txt installs cleanly with
uv + Python 3.12, resolving to tokenizers 0.22.2 and transformers 5.3.0.

All CLI commands documented in the README were tested successfully after
this change, including src/assets/run.py and src/benchmarks/run.py.

requirements.txt

@@ -14,7 +14,7 @@ pydantic>=2.0.0
 # Note: flash-attn must be installed separately - see README for instructions
 torch>=2.7.0
 transformers>=4.48.0
-tokenizers==0.20.3
+tokenizers>=0.20.3
 addict>=2.4.0
 easydict>=1.10
 einops>=0.6.0