fix: make sure head can be used in DeepPot (#4312)

Following the example mentioned
[here](https://www.aissquare.com/models/detail?pageType=models&name=DPA-2.3.0-v3.0.0b4&id=279)
, I first select a `head` and put it into `DPCalculator` but encounter
the error like this:
```shell
(/public/home/mzq001/soft/deepmd-kit) [mzq001@login01 ~]$ python test.py 
To get the best performance, it is recommended to adjust the number of threads by setting the environment variables OMP_NUM_THREADS, DP_INTRA_OP_PARALLELISM_THREADS, and DP_INTER_OP_PARALLELISM_THREADS. See https://deepmd.rtfd.io/parallelism/ for more information.
Traceback (most recent call last):
  File "/public/home/mzq001/test.py", line 4, in <module>
    dp = DPCalculator("DPA2_medium_28_10M_beta4.pt", head="H2O_H2O-PD")
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/public/home/mzq001/soft/deepmd-kit/lib/python3.11/site-packages/deepmd/calculator.py", line 92, in __init__
    self.dp = DeepPot(str(Path(model).resolve()), neighbor_list=neighbor_list)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/public/home/mzq001/soft/deepmd-kit/lib/python3.11/site-packages/deepmd/infer/deep_eval.py", line 334, in __init__
    self.deep_eval = DeepEvalBackend(
                     ^^^^^^^^^^^^^^^^
  File "/public/home/mzq001/soft/deepmd-kit/lib/python3.11/site-packages/deepmd/pt/infer/deep_eval.py", line 121, in __init__
    head is not None
AssertionError: Head must be set for multitask model! Available heads are: ['Domains_Alloy', 'Domains_Anode', 'Domains_Cluster', 'Domains_Drug', 'Domains_FerroEle', 'Domains_OC2M', 'Domains_SSE-PBE', 'Domains_SemiCond', 'H2O_H2O-PD', 'Metals_AgAu-PBE', 'Metals_AlMgCu', 'Metals_Cu', 'Metals_Sn', 'Metals_Ti', 'Metals_V', 'Metals_W', 'Others_C12H26', 'Others_HfO2', 'Domains_ANI', 'Domains_SSE-PBESol', 'Domains_Transition1x', 'H2O_H2O-DPLR', 'H2O_H2O-PBE0TS-MD', 'H2O_H2O-PBE0TS', 'H2O_H2O-SCAN0', 'Metals_AgAu-PBED3', 'Others_In2Se3', 'MP_traj_v024_alldata_mixu']
```

```python
## Compute potential energy
from ase import Atoms
from deepmd.calculator import DP as DPCalculator
dp = DPCalculator("DPA2_medium_28_10M_beta4.pt", head="H2O_H2O-PD")
water = Atoms('H2O', positions=[(0.7601, 1.9270, 1), (1.9575, 1, 1), (1., 1., 1.)], cell=[100, 100, 100])
water.calc = dp
print(water.get_potential_energy())
print(water.get_forces())

## Run BFGS structure optimization
from ase.optimize import BFGS
dyn = BFGS(water)
dyn.run(fmax=1e-6)
print(water.get_positions())
```

The variable named `head` of `DPCalculator` is not being used in
`DeepPot` which directly leads to this error.



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

- **New Features**
- Enhanced configurability of the DP class with an additional `head`
parameter for initialization.
- Updated default behavior in the calculate method to include stress
calculations alongside energy and forces.

- **Bug Fixes**
- Clarified handling of stress property in the context of lattice
relaxation, improving accuracy in calculations.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

deepmd/calculator.py

@@ -45,6 +45,8 @@ class DP(Calculator):
         will infer this information from model, by default None
     neighbor_list : ase.neighborlist.NeighborList, optional
         The neighbor list object. If None, then build the native neighbor list.
+    head : Union[str, None], optional
+        a specific model branch choosing from pretrained model, by default None
 
     Examples
     --------
@@ -84,10 +86,15 @@ def __init__(
         label: str = "DP",
         type_dict: Optional[dict[str, int]] = None,
         neighbor_list=None,
+        head=None,
         **kwargs,
     ) -> None:
         Calculator.__init__(self, label=label, **kwargs)
-        self.dp = DeepPot(str(Path(model).resolve()), neighbor_list=neighbor_list)
+        self.dp = DeepPot(
+            str(Path(model).resolve()),
+            neighbor_list=neighbor_list,
+            head=head,
+        )
         if type_dict:
             self.type_dict = type_dict
         else: