fix(jax): fix typo c_differentiable -> r_differentiable (#4640)

We haven't met a situation where  c_differentiable != r_differentiable.

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## Summary by CodeRabbit

- **Refactor**
- Refined internal processing to streamline evaluation logic. These
improvements optimize back-end calculations while ensuring consistent
performance and user experience.

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

Author: njzjz

deepmd/jax/model/base_model.py

@@ -47,7 +47,7 @@ def forward_common_atomic(
             kk_redu = get_reduce_name(kk)
             model_predict[kk_redu] = jnp.sum(vv, axis=atom_axis)
             kk_derv_r, kk_derv_c = get_deriv_name(kk)
-            if vdef.c_differentiable:
+            if vdef.r_differentiable:
 
                 def eval_output(
                     cc_ext,