test: eagerly import torch in conftest to fix test-api shard flake

[nicoloboschi]

Symptom

test-api shard 2/3 intermittently fails, with dozens of tests erroring at collection time:

RuntimeError: function '_has_torch_function' already has a docstring
../.venv/.../torch/overrides.py:1778: RuntimeError

The count drifts run-to-run (62 → 58 → 34 collection errors), and re-running sometimes clears it — a classic nondeterministic flake. It is unrelated to any individual PR; it just happens to land on whichever tests xdist schedules onto the affected worker.

Root cause

The first import torch in a worker process happens lazily, from inside concurrent/async code:

tests/conftest.py:415  embeddings fixture -> emb.initialize()
hindsight_api/engine/embeddings.py:174  from sentence_transformers import SentenceTransformer
   -> transformers -> import torch
   -> torch/_tensor.py -> torch/overrides.py:1778
        has_torch_function = _add_docstr(_has_torch_function, "...")   # <-- raises

cross_encoder.py similarly imports torch lazily from inside a ThreadPoolExecutor. torch's C-level _add_docstr(...) registration in torch/overrides.py is not re-entrancy-safe — if torch/overrides.py gets executed twice in one process (which happens when the first import is driven from concurrent/threaded code), the second execution raises because the docstring is already set. That kills collection of every test on the shard.

Fix

Import torch once, in the main thread, at conftest import time — before any fixture spins up an event loop or sentence-transformers' thread pools. This makes the _add_docstr registration happen exactly once per xdist worker process; every later lazy import torch is then just a sys.modules hit. Guarded with try/except ImportError so slim/no-torch environments still collect.

Test-only change (tests/conftest.py); no production code touched.

Verification

• ruff check clean (the # noqa: F401 keeps the intentional import).
• Local suite still collects and runs (test_link_utils.py: 40 passed).
• The flake is nondeterministic, so the real signal is repeated-green test-api shards in CI on this PR.