Modify MeanEnsemble to accumulate results in-place.

[staydelight]

Fixes #6366 .

Description

According to the description in issue 6366, modify MeanEnsemble.

Types of changes

• 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.

[KumoLiu]

Thanks for the PR, leave several comments.
And also please ensure the unittest pass, the error log shown here: https://github.com/Project-MONAI/MONAI/actions/runs/11289371448/job/31399089589?pr=8141#step:8:14508

[KumoLiu]

If self.weights is None, this line will show error. You may need set a default value or move the computation in the for-loop.

[KumoLiu]

You may need check the the type for img what if it's a single array. Something like the change here:

MONAI/monai/transforms/post/array.py

Lines 639 to 644 in bae8f59

	if isinstance(img, Sequence) and isinstance(img[0], np.ndarray):
	img = [torch.as_tensor(i) for i in img]
	elif isinstance(img, np.ndarray):
	img = torch.as_tensor(img)
	out: torch.Tensor = torch.stack(img) if isinstance(img, Sequence) else img # type: ignore
	return out

[KumoLiu]

Did you handle whether self.weights has the same shape with the img?