Refactor torch_coherence function to include linear detrend option

bnpm/spectral.py

@@ -611,8 +611,9 @@ def detrend_constant(y, axis):
         y = y - torch.mean(y, axis=axis, keepdim=True)
         return y
 
-    X_linearDetrendPrep = torch.ones(nfft, 2, dtype=y.dtype, device=y.device)
-    X_linearDetrendPrep[:, 1] = torch.arange(nfft, dtype=y.dtype, device=y.device)
+    if detrend == 'linear':
+        X_linearDetrendPrep = torch.ones(nfft, 2, dtype=y.dtype, device=y.device)
+        X_linearDetrendPrep[:, 1] = torch.arange(nfft, dtype=y.dtype, device=y.device)
     def detrend_linear(y, axis):
         """
         Uses least squares approach to remove linear trend.