dev(hansbug): add tensor method for treetensor.numpy.ndarray class

test/numpy/test_array.py

 import numpy as np
 import pytest
+import torch
 
 import treetensor.numpy as tnp
+import treetensor.torch as ttorch
 from treetensor.common import Object
 
 
@@ -233,3 +235,22 @@ class TestNumpyArray:
                 'd': [0, 0, 0.0],
             }
         })
+
+    def test_tensor(self):
+        assert self._DEMO_1.tensor() == ttorch.Tensor({
+            'a': ttorch.Tensor([[1, 2, 3], [4, 5, 6]]),
+            'b': ttorch.Tensor([1, 3, 5, 7]),
+            'x': {
+                'c': ttorch.Tensor([[11], [23]]),
+                'd': ttorch.Tensor([3, 9, 11.0])
+            }
+        })
+
+        assert self._DEMO_1.tensor(dtype=torch.float64) == ttorch.Tensor({
+            'a': ttorch.Tensor([[1, 2, 3], [4, 5, 6]], dtype=torch.float64),
+            'b': ttorch.Tensor([1, 3, 5, 7], dtype=torch.float64),
+            'x': {
+                'c': ttorch.Tensor([[11], [23]], dtype=torch.float64),
+                'd': ttorch.Tensor([3, 9, 11.0], dtype=torch.float64),
+            }
+        })

treetensor/numpy/array.py

+from functools import lru_cache
+
 import numpy
+import torch
 from treevalue import method_treelize
 
 from .base import TreeNumpy
@@ -12,6 +15,12 @@ __all__ = [
 _ArrayProxy, _InstanceArrayProxy = get_tree_proxy(numpy.ndarray)
 
 
+@lru_cache()
+def _get_tensor_class(args0):
+    from ..torch import Tensor
+    return Tensor
+
+
 class _BaseArrayMeta(clsmeta(numpy.asarray, allow_dict=True)):
     pass
 
@@ -92,6 +101,13 @@ class ndarray(TreeNumpy, metaclass=_ArrayMeta):
     def any(self: numpy.ndarray, *args, **kwargs):
         return self.any(*args, **kwargs)
 
+    @method_treelize(return_type=_get_tensor_class)
+    def tensor(self: numpy.ndarray, *args, **kwargs):
+        tensor_: torch.Tensor = torch.from_numpy(self)
+        if args or kwargs:
+            tensor_ = tensor_.to(*args, **kwargs)
+        return tensor_
+
     @method_treelize()
     def __eq__(self, other):
         """