Add unit tests for loading_data.py and model.py

[beta-devin-ai-integration]

Summary

Raises unit test coverage from a single test file to 24 tests, without modifying any source behavior. Branched off devin/1783062025-demo-prep so the CI workflow and existing tests are included.

• tests/test_loading_data.py — tests loading_FD001 two ways:
  • a synthetic_cmapss fixture that writes tiny synthetic train_FD001.txt / test_FD001.txt / RUL_FD001.txt files into a tmp dir and monkeypatch.chdirs there (since loading_FD001 reads ./CMAPSSData/ relative to cwd). Verifies return types, per-unit group sizes, dropped settings/sensors (s_1, s_5, s_6, s_10, s_16, s_18, s_19), min-max normalization to [0,1], piece-wise RUL (decreasing to 0, clipped at 125), y_test contents, and no RUL column on the test set.
  • a smoke test against the real CMAPSSData/ files (100 units each in train/test/RUL).
• tests/test_model.py — pure-logic tests using tiny configs (d_model=16, no training): forward output shapes for Transformer, Gating, Encoder, EncoderLayer, MultiHeadAttention, FeedForward, and the attention function; PositionalEncoder output verified against the sin/cos formula; get_clones independence; Norm zero-mean output; Gating.init_weights bounds; and a backward pass producing nonzero gradients.
• conftest.py — adds a repo_root fixture (was an empty file; no behavior change).
• .github/workflows/ci.yml:
  • installs CPU-only torch (pip install torch --index-url https://download.pytorch.org/whl/cpu) so the model tests run in CI.
  • pins pandas<3: pandas 3 silently no-ops the chained train_norm['RUL'].clip(upper=125, inplace=True) in loading_data.py, so RUL is no longer clipped. The repo's requirements.txt pins pandas 1.x, so pinning CI (rather than changing source behavior) keeps this PR test-only.

No source files were modified.

Link to Devin session: https://cognition.beta.devinenterprise.com/sessions/a50633ee4f7d422eaed79d5271b084d6
Requested by: @claudiahubert

---

Devin Review

Status	Commit
⚪ Not started	—

Run Devin Review

[beta-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring