[Feature] Support modulated_deform_conv with cambricon MLU backend

[mengpenghui]

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Motivation

The motivation of the PR is to support running modulated_deform_conv with Cambricon MLU backend.

It includes two parts:

1. Refactor modulated_deform_conv.py to support MLU backend.
2. Refactor test_modulated_deform_conv.py to support testing modulated_deform_conv with MLU backend.

Modification

• PyTorch adaptation
  Adapt modulated_deform_conv for PyTorch in :
  mmcv/ops/modulated_deform_conv.py
mmcv/ops/__init__.py

• Unit test
  Support test modulated_deform_conv with various backends in :
  tests/test_ops/test_modulated_deform_conv.py

• Documentation
  Added modulated_deform_conv to operator support document of mlu in :
  docs/en/understand_mmcv/ops.md
docs/zh_cn/understand_mmcv/ops.md

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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.

[CLAassistant]

All committers have signed the CLA.

[OpenMMLab-Assistant001]

Hi @mengpenghui ~

First of all, we want to express our gratitude for your significant PR in the MMCV project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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