Make mask and where distributed

[goodwanghan]

When using Ray backend, I can see the warning that mask and where default to pandas. Does that mean the operation will cause modin to collect all data onto one machine and then use pandas to do the job? If so, does it collect only the specific columns or it collects the whole dataframe onto one machine?

mask and where are relatively common operations, can we make them distributed?

Thank you!

[mvashishtha]

@goodwanghan thank you for your request. where is already implemented in a distributed way. We'll track mask implementation in #1206.