Support encoded RLE format in for COCO segmentations

[NicolasHug]

Closes #8351

This lets our COCO wrapper support segmentations that are already in "encoded RLE" format.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8387

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❌ 11 New Failures, 8 Unrelated Failures

As of commit 2f7c1a5 with merge base 26af015 ():

NEW FAILURES - The following jobs have failed:

• CMake / windows (windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• CMake / windows (windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-linux (3.11, linux.12xlarge, cpu) / linux-job (gh)
  FAILED
• Tests / unittests-macos (3.11, macos-m1-stable) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Tests / unittests-macos (3.12, macos-m1-stable) / macos-job (gh)
  RuntimeError: operator torchvision::nms does not exist
• Tests / unittests-windows (3.10, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.11, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.12, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.8, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.8, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.9, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• CMake / linux (linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Tests / unittests-linux (3.8, linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Build M1 Conda / pytorch/vision / upload / conda-py3_8-cpu (gh) (trunk failure)
• Build M1 Wheels / pytorch/vision / upload / wheel-py3_8-cpu (gh) (trunk failure)
  Unable to find any artifacts for the associated workflow
• CMake / macos (macos-m1-stable) / macos-job (gh) (trunk failure)
  RuntimeError: operator torchvision::nms does not exist
• Tests / unittests-macos (3.10, macos-m1-stable) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Tests / unittests-macos (3.8, macos-m1-stable) / macos-job (gh) (trunk failure)
  RuntimeError: operator torchvision::nms does not exist
• Tests / unittests-macos (3.9, macos-m1-stable) / macos-job (gh) (trunk failure)
  RuntimeError: operator torchvision::nms does not exist

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[NicolasHug]

Really not a fan of that test's logic, but I don't know how to better test that. @pmeier LMK if there are better options?