Fix distributed sampler initialization and exceeded sampler warning…

… false positives (#1270)

* Fix false positives about 'exceeded' warning

* Fix _maybe_init_distributed

lhotse/dataset/sampling/base.py

@@ -10,7 +10,7 @@
 
 from lhotse.cut import Cut, CutSet
 from lhotse.lazy import Dillable
-from lhotse.utils import Seconds, is_none_or_gt
+from lhotse.utils import Seconds, ifnone, is_none_or_gt
 
 
 class CutSampler(Sampler, Dillable):
@@ -101,17 +101,22 @@ def _maybe_init_distributed(self, world_size: Optional[int], rank: Optional[int]
             assert world_size >= 1
         if rank is not None:
             assert rank >= 0
+
+        # Order of precedence:
+        # 1. When world size or rank are explicitly provided, we will use them.
+        # 2. Next, check WORLD_SIZE and RANK env variables; yes? use them.
+        # 3. Next, check if torch.distributed is initialized and has them set; yes? use them.
+        # 4. If none of those are available, rank=0 and world_size=1.
         if "WORLD_SIZE" in os.environ and "RANK" in os.environ:
             # If deepspeed launcher is being used, it will set the env variables automatically.
-            self.world_size = int(os.environ["WORLD_SIZE"])
-            self.rank = int(os.environ["RANK"])
-            return
-        if not dist.is_available() or not dist.is_initialized():
-            self.world_size = 1 if world_size is None else world_size
-            self.rank = 0 if rank is None else rank
-            return
-        self.world_size = dist.get_world_size() if world_size is None else world_size
-        self.rank = dist.get_rank() if rank is None else rank
+            self.world_size = ifnone(world_size, int(os.environ["WORLD_SIZE"]))
+            self.rank = ifnone(rank, int(os.environ["RANK"]))
+        elif dist.is_available() and dist.is_initialized():
+            self.world_size = ifnone(world_size, dist.get_world_size())
+            self.rank = ifnone(rank, dist.get_rank())
+        else:
+            self.world_size = ifnone(world_size, 1)
+            self.rank = ifnone(rank, 0)
         assert self.rank < self.world_size
 
     def set_epoch(self, epoch: int) -> None:

lhotse/dataset/sampling/dynamic.py

@@ -322,9 +322,9 @@ def detuplify(
             # Did we exceed the max_frames and max_cuts constraints?
             if self.time_constraint.close_to_exceeding():
                 # Yes. Finish sampling this batch.
-                if self.time_constraint.exceeded():
+                if self.time_constraint.exceeded() and len(cuts) == 1:
                     warnings.warn(
-                        "We have exceeded the max_duration constraint during sampling. "
+                        "We have exceeded the max_duration constraint during sampling but have only 1 cut. "
                         "This is likely because max_duration was set to a very low value ~10s, "
                         "or you're using a CutSet with very long cuts (e.g. 100s of seconds long)."
                     )