Remove __len__ from CombinedStreamingDataset (#19321)

Author: awaelchli

src/lightning/data/streaming/combined.py

@@ -27,8 +27,11 @@ class CombinedStreamingDataset(IterableDataset):
     """The `CombinedStreamingDataset` enables to stream data from multiple StreamingDataset with the sampling ratio of
     your choice.
 
-    Addtionally, the `CombinedStreamingDataset` keeps track of the number of
-    samples fetched to enable resumability of the datasets.
+    Addtionally, the `CombinedStreamingDataset` keeps track of the number of samples fetched to enable resumability
+    of the datasets.
+
+    Note that due to the random sampling, the number of samples returned from the iterator is variable and a function
+    of the given seed. The combined dataset will raise a StopIteration as soon as any of the datasets is exhausted.
 
     """
 
@@ -71,10 +74,6 @@ def _set_use_streaming_dataloader(self, use_streaming_dataloader: bool) -> None:
         # Used to prevent returning num_samples_yielded when using PyTorch DataLoader
         self._use_streaming_dataloader = use_streaming_dataloader
 
-    def __len__(self) -> int:
-        assert self._weights
-        return int(min([1 / w * len(d) for w, d in zip(self._weights, self._datasets) if w > 0]))
-
     def __iter__(self) -> Iterator[Any]:
         assert self._weights