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

Co-authored-by: Xin Yao <xiny@nvidia.com>

examples/pytorch/dimenet/modules/bessel_basis_layer.py

@@ -17,7 +17,9 @@ class BesselBasisLayer(nn.Module):
         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
@@ -25,4 +27,4 @@ class BesselBasisLayer(nn.Module):
         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
\ No newline at end of file
+        return g