Add einops to base dependencies (fixes #212)

[jkukul]

Summary

einops is imported at module-load time on the core model path but is only declared in
the optional notebooks extra, so a base install (pip install . or
pip install git+https://github.com/facebookresearch/sam3.git) crashes on import sam3.

This moves einops into [project].dependencies so the package imports out of the box.

Fixes #212.

The import chain that triggers it

import sam3 → sam3/__init__.py → model_builder → model/decoder →
sam/transformer → sam/rope.py, which does:

from einops import rearrange, repeat

This is on the default inference path (RoPE), not behind any use_fa3 / notebook / fast-kernel guard, so every consumer needs einops.

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