Add support of TorchVision's Model Registration API on MMCV

[datumbox]

Hi OpenMMLab team,

First of all thank you very much for developing your awesome framework.

TorchVision has recently introduced a new Model Registration API which allows the easier listing of models and weights. Its aim is to eliminate the somehow hacky idiom of using torchvision.models.__dict__ to retrieve models and weights.

In this PR, I'm updating your existing implementation introduced at #1848 to use our new API. Please note that I'm going for the minimum number of modifications necessary; the code may be further simplified but that might require additional discussion.

For full transparency, the new API is currently considered Beta. We've intentionally marked it as such to ensure that we can act upon the feedback of our partners and users. We are currently collecting feedback at pytorch/vision#6365 and hoping to promote to stable before TorchVision v0.14. If you have any feedback or feature requests, we would be happy to discuss more. Finally, please note that if we end up making breaking changes on the API after you merge this PR, we will make sure to follow up with all the necessary fixes to keep the Open-MMLab users unaffected.

Looking forward to your feedback.

[CLAassistant]

All committers have signed the CLA.

[zhouzaida]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.You have signed the CLA already but the status is still pending? Let us recheck it.

Hi @datumbox , thanks to the TorchVision team for keeping us informed of new changes. Totally agree with the Model Registration API to make it easier to get models and weights. After signing the CLA, we can merge this PR.

[datumbox]

@zhouzaida Thank you very much for the prompt review. I really appreciate it.

I've raised a request to my company to approve my signing of the CLAs. It's likely to take a week but I don't believe this is going to be an issue. I opted for going through the whole approval process to be able to make more contributions to your project on the future if necessary. :)

I'll ping you once I get the approval and sign the CLAs.

[zhouzaida]

@zhouzaida Thank you very much for the prompt review. I really appreciate it.

I've raised a request to my company to approve my signing of the CLAs. It's likely to take a week but I don't believe this is going to be an issue. I opted for going through the whole approval process to be able to make more contributions to your project on the future if necessary. :)

I'll ping you once I get the approval and sign the CLAs.

Okay, looking forward to our more cooperation.

[datumbox]

@zhouzaida I got permission to sign the CLAs. We should be good to go :)

Let me know if you need anything else from my side. Thanks!

[OpenMMLab-Assistant005]

Hi @datumbox ！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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