datasets: [GSoC] Tests are failing for PlantVillage and The300wLpTest

From: https://source.cloud.google.com/results/invocations/9d95e4db-01a3-407e-9afc-3c7105f13740/targets/tensorflow_datasets%2Fgh_testing%2Fcontinuous/log

It seems that some tests are failing for TFDS used with tf-nightly:

PlantVillage:

tensorflow_datasets/testing/dataset_builder_testing.py:263: in test_download_and_prepare_as_dataset
    self._download_and_prepare_as_dataset(self.builder)
tensorflow_datasets/testing/dataset_builder_testing.py:302: in _download_and_prepare_as_dataset
    self._assertNumSamples(builder)
tensorflow_datasets/testing/dataset_builder_testing.py:343: in _assertNumSamples
    expected_num_examples,
E   AssertionError: 35 != 38

The300wLpTest:

num_examples = 0, number_of_shards = 1

    def _get_shard_boundaries(num_examples, number_of_shards):
      if num_examples == 0:
>       raise AssertionError("No examples were yielded.")
E       AssertionError: No examples were yielded.

See full logs at: https://source.cloud.google.com/results/invocations/80b640f7-df77-44bb-a034-0ddab98db139/targets

To reproduce: pytest -n auto --disable-warnings path/to/dataset_test.py (with pytest-xdist installed)

TF 1.15 and TF 2.1 works fine, which seems to indicates a regression in TensorFlow.