Add native TEST phase to TorchTNT framework

[zedsdead01]

Summary:
Adds a native TEST phase to TorchTNT as a first-class citizen, following the existing patterns for TRAIN, EVALUATE, and PREDICT phases. This eliminates the need for workarounds that repurpose PREDICT for testing (e.g., Vizard's D88975254).

The TEST phase is semantically distinct from PREDICT (which only runs forward passes) - it runs evaluation on held-out test data with loss/metrics computation, similar to EVALUATE.

Changes made:

State and Enums:

• Added EntryPoint.TEST and ActivePhase.TEST to state.py
• Added Phase.TEST to utils/checkpoint.py
• Added test_state property to State class
• Updated active_phase_state() and into_phase() to handle TEST

TestUnit Interface (unit.py):

• Added TestUnit ABC with test_step, lifecycle hooks (on_test_start, on_test_epoch_start, on_test_epoch_end, on_test_end), and get_next_test_batch
• Added TTestData TypeVar and TTestUnit type alias

Callback Hooks (callback.py, _callback_handler.py):

• Added 10 on_test_* callback hooks mirroring the eval/predict patterns
• Added corresponding dispatcher methods in CallbackHandler

Test Loop (test.py - new file):

• Created test() entry point and _test_impl() loop, modeled after predict.py
• Uses torch.no_grad() context (like evaluate, not inference_mode like predict)
• Handles module eval mode, callback dispatch, progress tracking, and exception handling

AutoUnit Integration (auto_unit.py):

• Added TestUnit[TData] to AutoUnit's inheritance chain
• Added test_step() (calls compute_loss, like eval_step)
• Added on_test_step_end() hook
• Added get_next_test_batch() with prefetch support
• Updated prefetch dicts to include ActivePhase.TEST

fit() Integration (fit.py):

• Added optional test_dataloader and max_test_steps_per_epoch parameters
• Test phase runs after training completes (not interleaved)
• Added type check for TestUnit when test_dataloader is provided

Exports and Test Utilities:

• Updated __init__.py to export test, TestUnit, TTestUnit
• Added DummyTestUnit, DummyFitTestUnit, and get_dummy_test_state to _test_utils.py

Build System:

• Added test library target to framework/BUCK
• Updated framework and fit targets to depend on test
• Added test_test test target to tests/framework/BUCK

Differential Revision: D96203908

[meta-codesync]

@zedsdead01 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D96203908.