6522 update __format__ impl of MetaTensor (#6523)

Fixes #6522


### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: Wenqi Li <wenqil@nvidia.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

monai/data/meta_tensor.py

@@ -590,6 +590,12 @@ def __str__(self):
         """
         return f"meta{str(self.as_tensor())}"
 
+    def __format__(self, format_spec):
+        """
+        returns the output of pytorch tensor's ``__format__`` method.
+        """
+        return self.as_tensor().__format__(format_spec)
+
     def print_verbose(self) -> None:
         """Verbose print with meta data."""
         print(self)

tests/test_meta_tensor.py

@@ -433,6 +433,7 @@ def test_str(self):
         t = MetaTensor([1.0], affine=torch.tensor(1), meta={"fname": "filename"})
         self.assertEqual(str(t), "metatensor([1.])")
         self.assertEqual(t.__repr__(), "metatensor([1.])")
+        self.assertEqual(f"{t[0]:.2f}", "1.00")
 
     def test_shape(self):
         s = MetaTensor([1])