What does `torch/distributed/fsdp/flat_param.py", line 1999, in _check_storage_allocated p_assert(storage_size > 0, "Expects storage to be allocated")` assertion do?

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What does torch/distributed/fsdp/flat_param.py", line 1999, in _check_storage_allocated p_assert(storage_size > 0, "Expects storage to be allocated") assertion do?

What can be the possible causes for storage size being 0?