address Copilot review: memoize YAML loads, add debug logging, fix test name

- Cache resolved YAML files in _resolve_if_path to avoid redundant
  disk/network I/O for sibling groups referencing the same file
- Add logging.debug when a path cannot be resolved, so failures
  are never fully silent
- Rename misleading test to match its assertion (total depth is 2,
  not 1; the unresolvable string is treated as a leaf)

Signed-off-by: Xuesong Yang <1646669+XuesongYang@users.noreply.github.com>

Author: XuesongYang

nemo/collections/common/data/lhotse/cutset.py

@@ -499,13 +499,19 @@ def count_input_cfg_levels(config: Union[DictConfig, dict]) -> int:
         2
     """
 
+    _cache: dict[str, object] = {}
+
     def _resolve_if_path(val):
         """If *val* is a string/Path, try to load the YAML it points to."""
         if isinstance(val, (str, Path)):
-            try:
-                return load_yaml(str(val))
-            except Exception:
-                return val
+            key = str(val)
+            if key not in _cache:
+                try:
+                    _cache[key] = load_yaml(key)
+                except Exception:
+                    logging.debug("count_input_cfg_levels: could not load %r, treating as leaf", key)
+                    _cache[key] = val
+            return _cache[key]
         return val
 
     def _max_depth(obj) -> int:

tests/collections/common/test_lhotse_temperature_reweighting.py

@@ -422,8 +422,8 @@ def test_two_level_string_input_cfg(self, tmp_path):
         config = {"input_cfg": str(top_yaml)}
         assert count_input_cfg_levels(config) == 2
 
-    def test_unresolvable_string_input_cfg_counts_as_one(self):
-        """Unresolvable path (e.g. OmegaConf interpolation) counts as 1 level."""
+    def test_unresolvable_nested_string_input_cfg_is_treated_as_leaf(self):
+        """Nested unresolvable string input_cfg is treated as a leaf, so total depth is 2."""
         config = {
             "input_cfg": [
                 {