Fix edge case of empty embedding tables

Summary:
ATT
Currently python interface for fbgemm doesn't support grouped embedding tables with all empty rows, due to the hash size calculation (log(0)). This just adds a special case for they're all empty, set hash_size_cumsum to be zero.

Should be pretty low risk as currently no one uses this module that hits that conditional.

Reviewed By: colin2328, xing-liu

Differential Revision: D32514857

fbshipit-source-id: a0cd1d282c8f48ebe8cd390b6a4bca227a79fdf1

fbgemm_gpu/fbgemm_gpu/split_table_batched_embeddings_ops.py

@@ -294,7 +294,10 @@ def __init__(  # noqa C901
         )
 
         hash_size_cumsum = [0] + list(accumulate(rows))
-        self.total_hash_size_bits = int(log2(float(hash_size_cumsum[-1])) + 1)
+        if hash_size_cumsum[-1] == 0:
+            self.total_hash_size_bits: int = 0
+        else:
+            self.total_hash_size_bits: int = int(log2(float(hash_size_cumsum[-1])) + 1)
         # The last element is to easily access # of rows of each table by
         # hash_size_cumsum[t + 1] - hash_size_cumsum[t]
         hash_size_cumsum = [hash_size_cumsum[t] for t in self.feature_table_map] + [
@@ -1362,7 +1365,10 @@ def __init__(
         assert self.D_offsets.numel() == T + 1
 
         hash_size_cumsum = [0] + list(accumulate(rows))
-        self.total_hash_size_bits = int(log2(float(hash_size_cumsum[-1])) + 1)
+        if hash_size_cumsum[-1] == 0:
+            self.total_hash_size_bits: int = 0
+        else:
+            self.total_hash_size_bits: int = int(log2(float(hash_size_cumsum[-1])) + 1)
         # The last element is to easily access # of rows of each table by
         # hash_size_cumsum[t + 1] - hash_size_cumsum[t]
         hash_size_cumsum = [hash_size_cumsum[t] for t in feature_table_map] + [