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This commit adds an optional argument in the `random_walk` function,
namely `return_edge_indices`. The default behaviour is not changed, but
if a user wants to directly use the edges visited by the random walker,
we can return the indices of those edges by setting
`return_edge_indices` to `True`. New cases are also added to the test
suite.

* half support

* deprecation

* typo

* test half

* fix test