Support TFDS format for segmentation

[hadusam]

What this patch does to fix the issue.

Close #1002

Link to any relevant issues or pull requests.

Refs

I referred to the official tutorial of segmentation to decide the TFDS feature format. (image, label, segmentation_mask)
https://www.tensorflow.org/tutorials/images/segmentation#download_the_oxford-iiit_pets_dataset

tfds.core.DatasetInfo(
    name='oxford_iiit_pet',
    version=3.0.0,
    description='The Oxford-IIIT pet dataset is a 37 category pet image dataset with roughly 200
images for each class. The images have large variations in scale, pose and
lighting. All images have an associated ground truth annotation of breed.
',
    urls=['http://www.robots.ox.ac.uk/~vgg/data/pets/'],
    features=FeaturesDict({
        'file_name': Text(shape=(), dtype=tf.string),
        'image': Image(shape=(None, None, 3), dtype=tf.uint8),
        'label': ClassLabel(shape=(), dtype=tf.int64, num_classes=37),
        'segmentation_mask': Image(shape=(None, None, 1), dtype=tf.uint8),
    }),
    total_num_examples=7349,
    splits={
        'test': 3669,
        'train': 3680,
    },
    supervised_keys=('image', 'label'),
    citation="""@InProceedings{parkhi12a,
      author       = "Parkhi, O. M. and Vedaldi, A. and Zisserman, A. and Jawahar, C.~V.",
      title        = "Cats and Dogs",
      booktitle    = "IEEE Conference on Computer Vision and Pattern Recognition",
      year         = "2012",
    }""",
    redistribution_info=,
)

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[tfujiwar]

Readability Approval
Thanks for expanding TFDS datasets!!

[tfujiwar]

So the feature label is used for having a list of classes but not needed for every example. Is my understanding correct?

[hadusam]

@tfujiwar
Yes! Your understanding is correct! 👍

[tfujiwar]

Thank you!

[hadusam]

I checked and compared trainings with TFDS and without TFDS(local image files on directories) with my sample config. (CamVid with IMAGE_SIZE = [160, 240])
This looks the same between them 👍

[iizukak]

OA

[iizukak]

/ready

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