Fix 'backend' usage in DPLR kernel

models/s4/s4.py

@@ -1375,9 +1375,9 @@ def forward(self, state=None, rate=1.0, L=None):
         v = v * dt  # Incorporate dt into B
 
         # Dispatch which Cauchy kernel to use
-        if has_cuda_extension and z.dtype == torch.cfloat and z.device.type == 'cuda' and self.kernel == 'cuda':
+        if has_cuda_extension and z.dtype == torch.cfloat and z.device.type == 'cuda' and self.backend == 'cuda':
             cauchy_mult = cauchy_cuda
-        elif has_pykeops and self.kernel in ['cuda', 'keops']:
+        elif has_pykeops and self.backend in ['cuda', 'keops']:
             cauchy_mult = cauchy_keops
         else:
             cauchy_mult = cauchy_naive