Add GTSRB dataset to the list of supporting datasets

[sumukhaithal6]

Addresses GTSRB dataset in #5108.
This PR adds the GTSRB dataset to torchvision.datasets module.

1. Adds support to GTSRB dataset with support to directly download the dataset.
   Note: No validation split in this dataset.
2. Adds corresponding unittests for the dataset.

Any suggestions/ feedback welcome.
cc: @pmeier @NicolasHug

cc @pmeier

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💊 CI failures summary and remediations

As of commit 297dfd2 (more details on the Dr. CI page):

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• 1/4 failures introduced in this PR
• 3/4 broken upstream at merge base 578c154 since Jan 04

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1 failure not recognized by patterns:

Job	Step	Action
unittest_macos_cpu_py3.8	Run tests	🔁 rerun

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🚧 3 ongoing upstream failures:

These were probably caused by upstream breakages that are not fixed yet.

• unittest_linux_cpu_py3.7 since Jan 04 (cc7e856)
  • 🔁 rerun
• unittest_linux_cpu_py3.9 since Jan 04 (cc7e856)
  • 🔁 rerun
• unittest_linux_cpu_py3.8 since Jan 04 (cc7e856)
  • 🔁 rerun

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[pmeier]

Thanks a lot @sumukhaithal6, this LGTM! Some minor nits inline.

[pmeier]

Some more nits to appease mypy.

[NicolasHug]

Thanks a lot @sumukhaithal6 for the PR, and @pmeier for the review!

I made a few comments below, but this looks great already. I realize this has been a while since the last approval so if you'd like me to address these comments myself, just let me know.

[NicolasHug]

CI failure are unrelated so I merged.

Thanks a ton @sumukhaithal6 and @pmeier !