Default download location

[cifkao]

There could be a default location where datasets are downloaded, like with TensorFlow Datasets. This should be possible to configure (.muspyrc?).

[salu133445]

We follow torchvision to have root directory as an argument. I am not sure how much efforts need to make this works as expected. A super naive implementation is to provide a default value for path if it is not given. For example, we could download datasets into $HOME/.muspy (configurable) and use a pattern like $HOME/.muspy/{module_name}/{class_name} for different datasets, e.g., $HOME/.muspy/maestro/MAESTRODatasetV1 for the MAESTRO Dataset v1 (the full path is somewhat ugly though).

I know tfds and mirdata both have a centralized directory for storing downloaded datasets. We could follow them if we want to add this feature.