Implement `return_counts` and `return_index` for `unique`

[neosunhan]

Feature functionality
numpy.unique provides 2 additional flags that are not present in heat.unique.

1. return_counts returns the number of times each unique item appears in the original array.
2. return_index returns the first occurrences of the unique items in the original array.

Additional context

• unique_all in the Python array API standard requires these flags to return the indices and counts of the unique values.
• Also see unique_counts in the Python array API standard

[github-actions]

Branch features/1021-Implement_return_counts_and_return_index_for_unique created!

[github-actions]

This issue is stale because it has been open for 60 days with no activity.

[github-actions]

This issue was closed because it has been inactive for 60 days since being marked as stale.