[Docs] Add model structure graph of YOLOv7 in configs\yolov7\README.md

[Audrey528]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Understand model structure of YOLOv7 clearly and quickly.

Modification

add model structure graph of YOLOv7 in configs\yolov7\README.md.

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 potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a potential influence on downstream projects, this PR should be tested with downstream projects, like MMDetection or MMClassification.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[RangeKing]

LGTM

[jason102811]

Hi @Audrey528 ！First of all, we want to express our gratitude for your significant PR in the MMYolo . 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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Thank you again for your contribution❤