fix(jax): fix repflows JIT issues (#4775)

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## Summary by CodeRabbit

- **Bug Fixes**
- Improved compatibility with just-in-time (JIT) compilation in certain
scenarios, preventing potential errors during execution. End-user
functionality remains unchanged.

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Signed-off-by: Jinzhe Zeng <jinzhe.zeng@ustc.edu.cn>

Author: njzjz

deepmd/dpmodel/descriptor/repflows.py

@@ -1328,7 +1328,10 @@ def call(
         )
         nb, nloc, nnei = nlist.shape
         nall = node_ebd_ext.shape[1]
-        n_edge = int(xp.sum(xp.astype(nlist_mask, xp.int32)))
+        # int cannot jit; do not run it when self.use_dynamic_sel == False
+        n_edge = (
+            int(xp.sum(xp.astype(nlist_mask, xp.int32))) if self.use_dynamic_sel else 0
+        )
         node_ebd = node_ebd_ext[:, :nloc, :]
         assert (nb, nloc) == node_ebd.shape[:2]
         if not self.use_dynamic_sel: