fix: complete dipole model backend conversion from TF to PT

- Fixed NetworkCollection serialization to handle None networks properly
- Added missing @variables and sel_type to DipoleFittingSeA serialization
- All tests pass and backend conversion works successfully
- Fixes #3672

Co-authored-by: njzjz <9496702+njzjz@users.noreply.github.com>

Author: Copilot

system/set.000/aparam.npy

@@ -0,0 +1,6 @@
+0
+0
+1
+1
+1
+1

system/set.000/box.npy

@@ -0,0 +1,2 @@
+foo
+bar