fix:fix LAMMPS MPI tests with mpi4py 4.0.0 (deepmodeling#4032)

The previous code works with mpi4py<4 but fails with mpi4py 4.0.0. I
don't know what breaking change was made.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Optimized potential energy calculation in the LAMMPS simulation by
restricting evaluation to the master process, reducing unnecessary
computations.

- **Chores**
- Improved control flow for better performance in parallel execution
contexts.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <[email protected]>

source/lmp/tests/run_mpi_pair_deepmd.py

@@ -54,8 +54,8 @@
 )
 lammps.pair_coeff("* *")
 lammps.run(0)
-pe = lammps.eval("pe")
 if rank == 0:
+    pe = lammps.eval("pe")
     arr = [pe]
     np.savetxt(output, np.array(arr))
 MPI.Finalize()