[FEA] Support missing operators in cudf.Series (__divmod__, __rdivmod__, __round__).

[bdice]

Is your feature request related to a problem? Please describe.
cudf.Series does not have reverse binary operators for bitwise operators like __rxor__. This was caught after changing the output of Series.hash_values from a cupy.array to a cudf.Series in #9390, which broke NVTabular: NVIDIA-Merlin/NVTabular#1376 (review). The cupy.array class supports both directions, like array ^ scalar and scalar ^ array. Similarly, pandas supports both directions. Currently cudf.Series only supports series ^ scalar.

Describe the solution you'd like
Add reverse binary operators for all cases that are missing, which include at least bitwise operators like __rxor__ but possibly others as well. Missing operators have been addressed several times in the past (#208, #213, #1292, #8598, ...). Resolving this issue should involve checking the full list in the Python object model documentation to make sure we catch everything that pandas supports.

Additional context
A workaround is to use the binary operator in the reverse order (for commutative operators).

[bdice]

List of other operators I've found are implemented in pandas but not cudf:

• __divmod__(self, other) (docs on divmod)
• __rdivmod__(self, other)
• __round__(self, ndigits)

[vyasr]

#10360 handles the reverse binary operators. The three that you listed in your last comment are not yet done. divmod and rdivmod are out of scope for #10360 but are in general trivial to implement in the relevant classes once that PR is merged since they are just

def __divmod__(self, other):
    return self // other, self % other
def __rdivmod__(self, other):
    return other // self, other % self

I think they should probably just bypass the new binop machinery, but maybe there will be a benefit to including them.

round is a little different from other binary operations because the second parameter is optional, i.e. round(x) will not pass a parameter for ndigits so you have to make sure to handle the None case. Obviously trivial to do, but a reason to perhaps not shoehorn it into the same patterns and instead implement it separately.

[bdice]

@vyasr Awesome. I subscribed to notifications on #10360 and I can take a look at the missing operators after that PR is merged. I changed the title of this issue to reflect the change in scope.

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[github-actions]

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