Fix the bug when type_map has only one element (#1196)

* Update data.py

Numpy has a bit odd behavior I ran into. The method `ndarray.loadtxt()` when loading from array file of length 1 outputs only the array element, not array of length=1.

```python
import numpy as np

np.__version__
"1.20.3"

!cat array.raw
A

np.loadtxt("array.raw").tolist()
"A"
```

* satisfy suggestions

Author: marian-code

deepmd/utils/data.py

@@ -508,7 +508,7 @@ def _make_idx_map(self, atom_type):
     def _load_type_map(self, sys_path: DPPath) :
         fname = sys_path / 'type_map.raw'
         if fname.is_file() :            
-            return fname.load_txt(dtype=str).tolist()
+            return fname.load_txt(dtype=str, ndmin=1).tolist()
         else :
             return None