Decide checkpoint phase-awareness from the checkpoint name only

[arpitjain099]

Summary:

CheckpointPath._populate_from_str picks between the phase-aware and phase-naive regex by testing for train_step / eval_step / predict_step anywhere in the path string, which includes the user-supplied dirpath. A phase-naive checkpoint that happens to live under a directory containing one of those substrings gets routed onto the phase-aware regex, and there are two ways that goes wrong:

• Silent corruption. foo/eval_step_sweeps/epoch_2_step_1_acc=0.98 parses, but (?:_(\w+)=(...))? swallows _step_1_acc=0.98 as the metric, so the result has step={} and a metric named step_1_acc.
• Hard failure. foo/train_step_dir/epoch_2_step_1 (no metric) matches neither branch of the phase-aware regex and raises ValueError: Attempted to parse malformed checkpoint path.

Both are reachable from ordinary use. CheckpointPath.path produces exactly these strings, so the serialize then parse round-trip breaks; __setstate__ calls _populate_from_str, so unpickling a CheckpointPath raises, which matters because this class is broadcast across ranks; and get_latest_checkpoint_path / get_checkpoint_dirpaths / CheckpointManager discovery break for anyone whose output directory is named something like eval_step_sweeps.

The dirpath has no bearing on whether the checkpoint itself is phase-aware, so the fix looks at the final path component only.

Test plan:

Added two entries to the existing valid_paths table in CheckpointPathTest::test_from_str, one per failure mode above. test_from_str fails before the change and the file passes after: pytest tests/utils/test_checkpoint.py gives 28 passed on torch 2.13 CPU, Python 3.12.