Update udsource partitions method to include total partitions

[yhl25]

numaproj/numaflow-java#212

[IlyasFardaouix]

I've taken a look at the PR and the updated udsource_partitions method in pynumaflow/shared/asynciter.py. It seems like the method is now correctly calculating the total number of partitions. However, I'm a bit concerned about the potential edge case where the udsource_partitions method is called with an empty list of partitions. In that scenario, the total_partitions variable would be set to 0, which might not be the expected behavior. Could you clarify how you'd like to handle this edge case?

[vaibhavtiwari33]

Could you please create a PR to share your changes.