The `BatchSplittingSampler` cannot handle empty batches

[xichens]

🐛 Bug

When poisson sampling is used, empty batches can occur. However, the BatchSplittingSampler from batch_memory_manager.py, which is called when using the BatchMemoryManager, cannot handle empty batches and will throw an error.

To Reproduce

To reproduce it, see this colab link.

Expected behavior

The wrapped batch sampler should handle empty batches properly

Additional context

I think the issue is with this line When calling

            split_idxs = np.array_split(
                batch_idxs, math.ceil(len(batch_idxs) / self.max_batch_size)
            )

the batch_idxs can be an empty list since it is from a UniformWithReplacementSampler, but np.array_split does not expect the first arg to be empty.

[xichens]

A follow up issue related to this one. Even without calling the wrap_data_loader from BatchMemoryManager, the DPDataLoader has some issue handling empty batches in certain cases. Currently the wrapt_collate_with_empty function here creates empty tensors for empty batches. However, it only sets the shape but not the dtype for the empty tensors. By default, these empty tensors will be of float. But some modules expect a particular data type as input, for example, for the torch.nn.Embedding, the inputs should be of int or Long.

I think the wrapt_collate_with_empty should consider the dtypes as well as shapes of the actual sample when creating the empty tensors.