[Fix] Fix mmdet flops error

[zwhus]

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Motivation

Now, mmdet don't support compute flops for all configs. I add some functions and modify many configs to fix mmdet flops error.

Modification

1. I mainly modify some configs and some metafile
2. I add forward function to compute flops in cascade_roi_head, scnet_roi_head, htc_roi_head
3. I add a function parameter in most of dense heads

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
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

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[hhaAndroid]

@zwhus I have no doubts about the code modification of this pr. The core question is how to calculate flops in two-stage is the most reasonable? I'm not very sure.

[ZwwWayne]

@zwhus I have no doubts about the code modification of this pr. The core question is how to calculate flops in two-stage is the most reasonable? I'm not very sure.

We can do it in the later PR. In this PR, we mainly fix the issue of metafile.

[jason102811]

Dear zwhus,
First of all, we want to express our gratitude for your significant PR in the MMDet 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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Best regards！ @zwhus