[Feature] Support MultiScaleDeformableAttn with cambricon MLU backend

[ZShaopeng]

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Motivation

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

It includes five parts:

1. Add ms_deform_attn_mlu_kernel.mlu src code as MLU kernel.
2. Add ms_deform_attn_mlu.cpp to support launching kernel in PyTorch.
3. Modify common_mlu_helper.hpp to add common function.
4. Refactor multi_scale_deform_attn.py to support MLU backend.
5. Refactor test_ms_deformable_attn.py to support testing MultiScaleDeformAttn with MLU backend.

Modification

• MLU src code
  Add MLU src code of MultiScaleDeformAttn in directory:
  mmcv/ops/csrc/common/mlu/ms_deform_attn_mlu_kernel.mlu,
  mmcv/ops/csrc/common/mlu/common_mlu_helper.hpp.
• PyTorch adaptation
  Adapt MultiScaleDeformAttn for PyTorch in directory:
  mmcv/ops/csrc/pytorch/mlu/ms_deform_attn_mlu.cpp,
  mmcv/ops/multi_scale_deform_attn.py.
• Unit test
  Support test MultiScaleDeformAttn with various backends in test_ms_deformable_attn.py.

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.

[grimoire]

Unit test test_gradient_numerical gradcheck failed.

[grimoire]

LGTM