Use floor div in aev (#437)

* ../aten/src/ATen/native/BinaryOps.cpp:81: UserWarning: Integer division of tensors using div or / is deprecated, and in a future release div will perform true division as in Python 3. Use true_divide or floor_divide (// in Python) instead.

* Update aev.py
Co-authored-by: Farhad Ramezanghorbani <farhadrgh@users.noreply.github.com>

torchani/aev.py

@@ -238,7 +238,7 @@ def triple_by_molecule(atom_index1: Tensor, atom_index2: Tensor) -> Tuple[Tensor
     counts = unique_results[-1]
 
     # compute central_atom_index
-    pair_sizes = (counts * (counts - 1) / 2).long()
+    pair_sizes = counts * (counts - 1) // 2
     pair_indices = torch.repeat_interleave(pair_sizes)
     central_atom_index = uniqued_central_atom_index.index_select(0, pair_indices)