Add sample visualization option to the DataModule API

[aribornstein]

🚀 Feature

It would be great to have the ability to sample image, text, tabular and audio samples from the flash datamodule.

Motivation

In ImageClassificationData, TextClassificationData, TabularClassificationData etc it would be great to have an overridable .visualize() method that plots or visualizes a sample of data.

Pitch

datamodule = ImageClassificationData.from_folders(
train_folder="data/hymenoptera_data/train/",
valid_folder="data/hymenoptera_data/val/",
test_folder="data/hymenoptera_data/test/",
)

datamodule.visualize(train, num_samples=6)

[ 1, 2, 3,
4, 5, 6]

Alternatives

Open to other ideas.

Additional context

[t-vi]

Ideally, this could be either before or after augmentation / pipeline collation.

[edgarriba]

merged #228