Add expected shapes in tvtensors docs

[mantasu]

Closes #8261, closes #8260

These are just some minor suggestions, please feel free to close if the current behavior is desired as mentioned here.

cc @NicolasHug

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8262

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 2d3c99b with merge base 2ae6a6d ():

NEW FAILURE - The following job has failed:

• Lint / python-types / linux-job (gh)
  RuntimeError: Command docker exec -t d6ce13663bcf70cb1b7cd660ab7e7cde005913decc9b2282a810db9f6c6b24c3 /exec failed with exit code 1

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

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

• Build Windows Conda / pytorch/vision / upload / conda-py3_8-cpu (gh)
  Unable to find an artifact with the name: pytorch_vision__3.8_cpu_

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[NicolasHug]

Thank you for the PR and sorry for the late review @mantasu !