[Example][Bugfix] Fix dimenet example (dmlc#4219)

Co-authored-by: Xin Yao <[email protected]>

examples/pytorch/dimenet/modules/bessel_basis_layer.py

@@ -17,12 +17,14 @@ def __init__(self,
         self.reset_params()
 
     def reset_params(self):
-        torch.arange(1, self.frequencies.numel() + 1, out=self.frequencies).mul_(np.pi)
+        with torch.no_grad():
+            torch.arange(1, self.frequencies.numel() + 1, out=self.frequencies).mul_(np.pi)
+        self.frequencies.requires_grad_()
 
     def forward(self, g):
         d_scaled = g.edata['d'] / self.cutoff
         # Necessary for proper broadcasting behaviour
         d_scaled = torch.unsqueeze(d_scaled, -1)
         d_cutoff = self.envelope(d_scaled)
         g.edata['rbf'] = d_cutoff * torch.sin(self.frequencies * d_scaled)
-        return g
+        return g