Fds add num_partitions property to partitioners

[adam-narozniak]

Issue

The number of partitions is a piece of information that is associated with a partitioner. However, there’s no way to access the number of partitions in the current partitioner abstractions. It makes it impossible to implement the creation of all partitions to use concatenate_divisions and create plots.

Description

NaturalIdPartitioner does not have the number of partitions (no need for that since it’s equal to the number of unique ids from the column specified by a user).

Other partitioners have either num_partitions or partition_sizes. This makes the specification quite diverse.

Related issues/PRs

To be created (concatenate_divisions and plotting). This is a prerequisite for these PRs.

Proposal

Add an abstract property num_partitions to the Partitioner. Expect users to trigger partitioning (+pior checks on the correctness) to ensure the correctness of this num_partitions.

Explanation

This is the most flexible solution I see right now. It doesn't require additional attributes in each partitioner and, due to our lazy partitioning, enables it to be triggered manually.
The part of the correctness check has to happen prior to partitioning (and can't be done e.g. in init) because it's only possible when the dataset is assigned.

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