Remove shape dependence from narrow_by

Good for torchscript

map2map/models/narrow.py

@@ -5,9 +5,8 @@ import torch.nn as nn
 def narrow_by(a, c):
     """Narrow a by size c symmetrically on all edges.
     """
-    for d in range(2, a.dim()):
-        a = a.narrow(d, c, a.shape[d] - 2 * c)
-    return a
+    ind = [slice(None)] * 2 + [slice(c, -c)] * (a.dim() - 2)
+    return a[tuple(ind)]
 
 
 def narrow_cast(*tensors):