Datasets API Update: Add Extra Params and Improve Testing

[oorliu]

Adding two new parameters to the get_graph() method within the datasets API.

• default_direction allows users to get only undirected graph objects.
• weights will specify whether or not the cugraph.Graph object has an edge_attr field.

test_dataset.py has been updated to:

1. Add coverage for the new parameters
2. Removing fetching datasets from the web when it's unnecessary.

Docstrings have also been updated for clarity.

[codecov-commenter]

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[rlratzel]

Just a couple very minor changes, otherwise looks almost done.

[rlratzel]

@gpucibot merge