fix(hansbug): fix bug of keys, values and items && update unittest

[HansBug]

import treetensor.torch as ttorch
print(ttorch.Size({'a': [1, 2]}).items())

Will cause error like this

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "treevalue/tree/tree/tree.pyx", line 850, in treevalue.tree.tree.tree.TreeValue.items
  File "treevalue/tree/tree/tree.pyx", line 876, in treevalue.tree.tree.tree.TreeValue.items
TypeError: Argument '_type' has incorrect type (expected type, got _MetaClass)

[codecov]

Codecov Report

Merging #66 (4aaf82d) into main (74d9375) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files          35       35           
  Lines        2152     2152           
=======================================
  Hits         2136     2136           
  Misses         16       16           

Flag	Coverage Δ
unittests	99.25% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
treevalue/tree/tree/tree.pyx	100.00% <100.00%> (ø)

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