Mino simplifications to prototype dataset testings

[NicolasHug]

This PR contains a few simplifications to the prototype dataset test suite as discussed offline with @pmeier

[facebook-github-bot]

💊 CI failures summary and remediations

As of commit 14acd48 (more details on the Dr. CI page):

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• 1/1 failures introduced in this PR

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🕵️ 1 new failure recognized by patterns

The following CI failures do not appear to be due to upstream breakages:

unittest_linux_cpu_py3.7 (1/1)

Step: "Run tests" (full log | diagnosis details | 🔁 rerun)

/root/project/torchvision/io/video.py:406: Runt...log: [mov,mp4,m4a,3gp,3g2,mj2] moov atom not found

test/test_image.py::test_decode_png[L-ImageReadMode.GRAY-palette_pytorch.png]
test/test_image.py::test_decode_png[RGB-ImageReadMode.RGB-palette_pytorch.png]
  /root/project/env/lib/python3.7/site-packages/PIL/Image.py:946: UserWarning: Palette images with Transparency expressed in bytes should be converted to RGBA images
    "Palette images with Transparency expressed in bytes should be "

test/test_io.py::TestVideo::test_probe_video_from_memory
  /root/project/torchvision/io/_video_opt.py:423: UserWarning: The given buffer is not writable, and PyTorch does not support non-writable tensors. This means you can write to the underlying (supposedly non-writable) buffer using the tensor. You may want to copy the buffer to protect its data or make it writable before converting it to a tensor. This type of warning will be suppressed for the rest of this program. (Triggered internally at  /opt/conda/conda-bld/pytorch_1643011704494/work/torch/csrc/utils/tensor_new.cpp:998.)
    video_data = torch.frombuffer(video_data, dtype=torch.uint8)

test/test_io.py::TestVideo::test_read_video_timestamps_corrupted_file
  /root/project/torchvision/io/video.py:406: RuntimeWarning: Failed to open container for /tmp/tmpycnxe5sm.mp4; Caught error: [Errno 1094995529] Invalid data found when processing input: '/tmp/tmpycnxe5sm.mp4'; last error log: [mov,mp4,m4a,3gp,3g2,mj2] moov atom not found
    warnings.warn(msg, RuntimeWarning)

test/test_models.py::test_memory_efficient_densenet[densenet121]
test/test_models.py::test_memory_efficient_densenet[densenet169]
test/test_models.py::test_memory_efficient_densenet[densenet201]
test/test_models.py::test_memory_efficient_densenet[densenet161]
  /root/project/env/lib/python3.7/site-packages/torch/autocast_mode.py:162: UserWarning: User provided device_type of 'cuda', but CUDA is not available. Disabling
    warnings.warn('User provided device_type of \'cuda\', but CUDA is not available. Disabling')

test/test_models.py::test_inception_v3_eval

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[pmeier]

LGTM, thanks a lot @NicolasHug!