fix deprecated in inference benchmark (#192)

tools/inference-benchmark.py

@@ -76,7 +76,7 @@ print()
 # test batch mode
 print('[Batch mode]')
 species, coordinates = torch.utils.data.dataloader.default_collate(list(xyz))
-coordinates = torch.tensor(coordinates, requires_grad=True)
+coordinates.requires_grad_(True)
 start = timeit.default_timer()
 energies = nnp((species, coordinates))[1]
 mid = timeit.default_timer()
@@ -92,7 +92,7 @@ if parser.tqdm:
     xyz = tqdm.tqdm(xyz)
 for species, coordinates in xyz:
     species = species.unsqueeze(0)
-    coordinates = torch.tensor(coordinates.unsqueeze(0), requires_grad=True)
+    coordinates = coordinates.unsqueeze(0).detach().requires_grad_(True)
     energies = nnp((species, coordinates))[1]
     force = -torch.autograd.grad(energies.sum(), coordinates)[0]
 print('Time:', timeit.default_timer() - start)