BatchStratifiedSampler crashes with ZeroDivisionError when a domain_ratios entry has no matching rows

Start in roll/datasets/sampler.py at BatchStratifiedSampler.init, especially the normalization around lines 31-39, then run the provided FakeDataset reproduction. Done means an absent domain stays removed from the sampler configuration and list(iter(sampler)) completes without ZeroDivisionError.

BatchStratifiedSampler.__init__ (roll/datasets/sampler.py) already handles a domain in domain_ratios that has zero matching rows: it deletes the domain from both self.domain_indices and self.domain_ratios and prints "{key} is empty, delete in sampling." (lines 31-34).

Five lines later it undoes that: self.domain_ratios = {key: value / sum_values for key, value in domain_ratios.items()} (line 39) rebuilds from the original constructor argument, not from the pruned self.domain_ratios, so the just-deleted domain comes back. domain_list and domain_batch_num are built from that dict, so the empty domain gets a batch count. self.domain_indices is a defaultdict(list), so the next access to it for that domain (building domain_batch_capacities) silently creates an empty list instead of raising. By __iter__, that domain has a positive batch count and zero indices, and repeat_times = (total_required + len(indices) - 1) // len(indices) divides by zero.

Repro (dataset with only domains "a" and "b"):

ds = FakeDataset(["a"] * 10 + ["b"] * 10)
sampler = BatchStratifiedSampler(ds, domain_ratios={"a": 0.5, "b": 0.3, "c": 0.2}, batch_size=10, drop_last=True)
list(iter(sampler))  # ZeroDivisionError: integer division or modulo by zero

The print says "c is empty, delete in sampling", then sampler.domain_ratios still has c: 0.2 right after __init__ returns. Training can't start at all whenever a domain_ratios config names a domain that's absent from the current shard, which is exactly the case the deletion code was meant to handle.

Fix looks like reading from the already-pruned self.domain_ratios on line 39 instead of the constructor's domain_ratios argument. Happy to send a PR if useful.