[Fix] Fix the output of scatter on MLU device

[Qiza-lyhm]

Motivation

MMDP or MMDDP use origin scatter_gather to scatter Tensor to one or more device(s).
And the result of scatter_map should be a List of Tensor, each item is for one device.

However, the modified scatter_gather for MLUDP or MLUDDP only returns the pure Tensor.

It causes dimension errors when scatter this result again.

Modification

Change the output of scatter_map, thus the output is just the same size as GPU or CPU result.

BC-breaking (Optional)

No BC-breaking.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.