fix(pt): fix compute_output_stats_global when atomic_output is `N…

…one` (#4155)

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## Summary by CodeRabbit

- **Bug Fixes**
- Improved error handling by ensuring that the output data is not `None`
before processing, preventing potential runtime errors.

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

deepmd/pt/utils/stat.py

@@ -478,7 +478,7 @@ def compute_output_stats_global(
     std_atom_e = {}
     for kk in keys:
         if kk in stats_input:
-            if atomic_output.get_data()[kk].intensive:
+            if atomic_output is not None and atomic_output.get_data()[kk].intensive:
                 task_dim = stats_input[kk].shape[1]
                 assert merged_natoms[kk].shape == (nf[kk], ntypes)
                 stats_input[kk] = (